Is this homebrew Olympian race (including its subraces) balanced compared to the official published races?

Question

Is this homebrew 5e race (including its subraces) balanced compared to the official published races?

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Olympian

• Size: Medium
• Speed: 30 feet
• You gain proficiency in death saving throws.
• You learn the thaumaturgy cantrip.

Subraces:

Minervian:

• ASI: +2 intelligence and +2 wisdom
• You gain proficiency in arcana and history.
• You gain proficiency in one additional language of your choice.
• Starting at 9th level, you can cast legend lore once per long rest.

Hermian:

• ASI: +1 to three ability scores of your choice.
• You gain proficiency in Medicine, Thieves' Tools, and Sleight of Hand.
• You always know which way is north.
• Your speed increases by 10 feet.

Jovian:

• ASI: +1 to strength and +2 to constitution
• You gain resistance to lightning damage.
• You learn the shocking grasp cantrip.
• At level 5, you can cast lightning arrow once per short rest.
• At level 10, you gain a flying speed equal to your walking speed.
• At level 15, you can cast control weather once per long rest.

Neptunian:

• ASI: +1 to strength and +2 to constitution
• You gain resistance to cold damage.
• You learn the shape water cantrip.
• You gain a swimming speed equal to your walking speed.
• At level 7, you can cast tidal wave once per long rest.
• At level 15, you can cast tsunami OR control weather once per long rest.

Answer 1

The races are mostly fine

I would replace Tsunami with Control Water for the Neptunian. An AoE spell that does half damage on a successful save is mediocre for a Druid, but better than great for a Fighter¹.
The same is true for Lightning Arrow for Jovian, make it once per long rest.

Olympian

As David Coffron wrote, death saving throws should not matter much between magical healing and good encounter design, and Thaumaturgy is mostly for flavor, does not add much to power.
Base race is fine.

Minervian

While two +2 are very rare, only the Mountan Dwarf has it, this combination is much weaker, basically the second worst after +2 Wis / +2 Cha.
I would much rather pick +2 Wis / +1 Con (or Dex)², so you are way behind a Half-elf.

This also makes the two skills fine.

Legend Lore has expensive components, and is not combat related. It does not add much to your power, while flavorful.

Hermian

Again, way behind a Half-elf for anything that needs Cha, but I could make a decent Cleric or Wizard with it. Still, a Variant Human is better.
Overall interesting and versatile, but far from strong.

Jovian

Looks really similar to the Fire Genasi, you should tie the spells to an ability.

We have a much earlier fly speed precedent with Aaracokra, and Control Weather is mostly for flawor, so this is fine too.

Neptunian

Very similar to Jovian, while Shape Water is significantly weaker than Shocking Grasp. Tidal Wave is comparable in damage to a 16th level Dragonborn's breath, but they get to do it once per short rest. I think it is ok, considering how mediocre the rest is.

As mentioned at the beginning, replace Tsunami with Control Water.

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1) the expected damage of an 8th level area spell is 45,5 according to the DMG, but Tsunami only does 33 in the first round, even worse on subsequent rounds. A Druid can do better damage against mobs of weak enemies, and use his concentraion more effectively, but these weaknesses do not apply to a Fighter
2) You mostly need only 3 abilities: Attack, AC and Con. Some classes can merge the first two, but no-one needs both a high Int and Wis.

Answer 2

In short, they are a little too strong and don't quite fit design principles

First of all, adding proficiency to Death Saving Throws is a weird feature. In order for it to become useful, players have to fall in battle, which is something they should be trying to avoid in most cases (meaning it will rarely come up). However, this can drastically reduce the risk involved with certain activities. I'm not sure how to evaluate it's power level so I will consider on par with some of the situational Expertise features like Stonecunning for my analysis.

Now, I'll go through each subrace and discuss why they are more powerful than official options.

Minervian

1. ASIs. The only other race in the entire game to get +2 to two different abilities is the Mountain Dwarf and one is applied to Constitution (the only purely defensive ability)
2. Comparing with the Half-Elf. The Half-Elf is already one of the strongest races in the game and the only two features it gets are Skill Versatility and Fey Ancestry. Gaining proficiency in Death Saving Throws and 2 skills already compares to Half-Elf in its entirety.
3. Bonuses. On top of the already comparable race, you get a flavorful cantrip and legend lore. These are fairly minor bonuses.

Fixing the Minervian: Reduce one of its Ability Score Increases by one and this subrace is fairly balanced.

Hermian

1. ASIs. These ASIs are on par with the Triton from Volo's Guide to Monsters, but are much more versatile since you choose them. They are better than the Variant Human ASIs.
2. Skilled. The skills presented here are situational but you still get three of them. So far a little bit weaker than the Variant Human (the Skilled feat without selection for an extra ASI).
3. Bonuses. Knowing which way is north is entirely flavor and has almost no balance consideration. The 40 feet walking speed is a huge problem, though. So far, there are no official races with that speed and high speed characters are very powerful. With this high speed it easily eclipses the Variant Human (essentially combining a specific Skilled feat and part of the Mobile and Keen Mind feats) which is already considered one of the best races.

Fixing the Hermian: Reduce the bonus to walking speed by 5 feet to bring it in line with the Wood Elf. Then the skill proficiency compare to Mask of the Wild + Keen Senses + Elven Weapon Training. The Death Saving Throws compare to Fey Ancestry, and Trance and knowing north are both flavor bonuses (Trance is a bit better but the Hermian chooses its ability increases so it balances out roughly).

Jovian

1. ASIs: These ASIs are a bit weak since the +2 is applied to the only ability that is purely defensive. However this is similar to the Hill Dwarf, Lizardfolk, and Genasi so there is precedent.
2. Lightning: Lightning Resistance and shocking grasp draws similarities to the Fire Genasi which also gets elemental resistance and an attack cantrip. I'm assuming the spellcasting ability is Constitution (if not that should be specified and the cantrip becomes a bit weaker).
3. Spells: Since we are comparing to the Fire Genasi, look at the spells. Fire Genasi get a level 1 spell per long rest at level 3. The Jovian get way more (lightning arrow on a short rest is ridiculously strong). Also, races are not intended to scale to level 15. The latest an official race gets a new feature (ignoring racial feats) is level 5.
4. Bonuses: Death Saving Throws proficiency and thaumaturgy

Fixing the Jovian: Remove the extra spells. The bonuses are already about as good as burning hands once per day at 3rd level. Instead you could include them as a racial feat (although more balancing would have to be considered, see my "Statement on Trait Selection" later in this answer) or include weaker spells on level 3 and 5 in a vein similar to Drow Magic.

Neptunian

1. ASIs. Other than the fact that these feel repetitive, the same considerations for balance as with the Jovian.
2. Jovian similarities. This subrace falls to some of the same problems as the Jovian. Again it compares very closely to the Water Genasi (both have a swimming speed, an elemental resistance, and shape water). Like the Fire Genasi, the Water Genasi gets one extra spell at 3rd level (this time a level 2 non-combat spell). Races should not gain features at level 7 or level 15.
3. Bonuses: Death Saving Throws proficiency and thaumaturgy

Fixing the Neptunian: Pretty much the same as Jovian. If you want more powerful spells tied only to a race, they need to be racial feats such as Drow High Magic and Svirfneblin Magic (both found in Xanathar's Guide to Everything). While there are no feats with a level requirement in the game thus far, this could be introduced if necessary (although I'd be wary of making a feat more useful than a class feature. The few feats that are cause serious balance concerns).

Statement on Trait Selection

Giving skill proficiencies is not a particularly interesting racial feature in my opinion. Granting one specific skill (i.e. Keen Mind), or two of your choice (i.e. Skill Versatility) is okay because the class and background skill selection still feels meaningful but bogging down a race with 2-3 skill proficiencies is very limiting as there are a finite number of skills. Even Hunter's Lore for the Lizardfolk which provides a choice of 2 among 5 skills felt very restricting in this regard when I played one. Consider instead a more flavorful secondary benefit (look at Stonecunning, Fey Ancestry, or Halfling Luck for inspiration).

Racial traits should not scale beyond 5th level (and only then usually just with a couple spells). Classes are the aspect of design for scaling into higher tiers and set you apart from the rest of your kind. If your race scales, it draws attention away from your class features. Racial feats are one solution (as addressed earlier), but you may also want to consider race-specific subclasses. One can be found in the Swords Coast Adventurer's Guide where the Bladesinger is Elf and Half-Elf only. Such a subclass could allow you to explore what happens when a Jovian or Neptunian decides to focus on strengthening its inborn magic more than another of its class might normally.

Answer 3

These depart fairly significantly from the RAW races, so it's difficult to judge. In particular:

• +2/+2 stats are rare (only used by a single Dwarf subrace afaict), and only used when the rest of the benefits are fairly minimal. Minervian seems like it has some decent stuff.
• Spells, and other level-gated benefits, are only ever granted up to level 5 in RAW. The influence of a character's race doesn't seem to be intended to go further than that.

It might be useful to consult this guide on homebrewing races for 5e, which has a lot of useful advice gleaned from all the RAW races.