Yes, this is balanced
the normal sharpshooter feat is very strong. This is maybe a little weaker, but not that much, it compares fairly to other half-feats being even a little stronger than some of them, and is a tiny bit weaker than the updated, very similar version from the D&D One playtest material. All in all this is a perfectly fine feat to have.
The normal Sharpshooter feat is strong
In general, the Sharpshooter feat from the core rules is one of the most powerful feats for optimizing damage builds. There are several reasons for this:
- even on average you have about a 65% hit chance (better with magic weapons), which means you need a 8 or better to hit, and the mathematical effect of Sharpshooter would contribute about 2 points of damage, assuming you deal about 10 damage baseline
- you can opt when to take that damage trade-off and when not, and if you know what you are doing, can often estimate if your opponent has a high or low AC to make it worthwhile or not. Against low-AC targets, the damage uplift can be much higher
- damage uplift gets a lot better when you have advantage, improving to about 4 points on average, and there are various ways to get advantage on attacks, a common one is to have some way to see in darkness when the opponents cannot, and attack from being unseen
All these points are shared by Great Weapon Master, which is also an often-used feat, but Sharpshooter has several additional advantages going for it
- you can partially compensate the -5 with the Archery fighting style (which adds +2 to hit), softening it to -3
- removing range and cover disadvantages means that you can hit any opponent on a normal battlefield with a weapon like a longbow, without ever needing to move, without being blocked by opposing front-line fighters, without exposing yourself to opportunity attacks, and without being unable to reach opponents on high ledges, behind arrow slits or murder holes. You effectively get perfect ability to attack anyone you want at range, without downsides
Ranged combat is already strong in 5e, even with the range impairments. The only catch is if you get pulled into melee where you suffer disadvantage with it, but even if someone closes up to you, you can just drop your ranged weapon for free, then draw a melee weapon as a free object interaction and are you no worse off than another melee fighter.
Ranged combat with Sharpshooter, especially in a build optimized for it, is at the high end of the power curve for damage dealers in 5e.
Your feat does OK on numerical comparison
Because normal Sharpshooter is very good, if you compare another feat to normal Sharpshooter, you have slack for your feat being weaker than Sharpshooter, without being a bad feat. For example, even if you nerf Sharpshooter to only deal +5 extra damage instead of +10, it can be argued that it still is a worthwhile feat.
In your case, it would be possible to leverage the benefit of the +1 to an ability score, in combination for example with a 15 starting ability score, to improve your modifier by one and get +1 to damage and also +1 to hit (which is roughly worth another point of damage), so you on average would get an effective about +2 to damage, as you would from the -5/+10 tradeoff at baseline, making he change somewhat neutral initially.
I think may still be a bit weaker, as eventually you will max out your ability at 20 points, and then the -5/+10 still works and leads to overall higher damage you can reach, and for the other reasons mentioned above why the -5/+10 is good, but it is not a major nerf, and your feat will still be OK.
Looking at other half-feats
Your feat lacks one of the most defining features of Sharpshooter, it may also be useful to compare it to some other half-feats. Lets looking at a few that are also focused on combat and from the core rules
Tavern Brawler. This allows you to attack with unarmed strikes and improvised weapons using proficiency and a d4 damage dice, and as a bonus action intiate a grapple upon hit. I think this opens up some intersting builds, but a d4 is a lousy damage die compared to most any weapon, so that is still not great. The main benefit is grapple as a bonus action. Actions are very valuable, but bonus actions also compete with each other for the one bonus action slot, whereas the loss of cover and range disadvantages is always on, with no opportunity cost, and means a net improvment of your to hit chance of at least +2 in most cases. I think this is at least on par.
Weapon Master. This feat gives you proficiency in four weapons of your choice. I think this is clearly weaker than your feat: most characters get proficiency in the weapons they need from their class already, and there are other options like the elf race to get proficieny in longswords and longbows, or some monk's Dedicated Weapon feature from Tasha's Cauldron of Everything. So this benfit would seem mostly may be for edge cases. In fact, some maintain the feat is awful for anyone, so this provides a good lower floor how weak a feat can get while still falling into the range of what the rules offer.
Looking at these, half-feats can have a range of side benefits on top of the ability score increase, but for the most part, you often will take them to get to an even ability score and with that a better ability bonus. For that, getting rid of cover and range disadvantage as side effects, seems to be a solid choice.
Comparison to D&D One Expert Class Package
The Unearthed Arcana Expert Class package redefines this feat in nearly the same way as you did
Prerequisite: Proficiency with Any Martial Weapon
You can make shots that others find impossible, granting you the following benefits:
Ability Score Increase. Increase your Dexterity score by 1, to a maximum of 20.
Bypass Cover. Your Ranged Attacks with Weapons ignore Half Cover and Three Quarters Cover.
Firing in Melee. Being within 5 feet of an enemy doesn’t impose Disadvantage on your ranged Attack Rolls with Weapons.
Long Shots. Attacking at Long Range doesn’t impose Disadvantage on your ranged Attack Rolls with Weapons.
This is exactly your feat, with the added bonus of not having disadvantage when firing in melee, which makes it even a bit stronger than yours. We have a sharpshooter ranger in our group, and over the course of about 13 levels of play, that situation has maybe come up 2 or 3 times, so it is quite rare for it to matter in practical play (although he might have taken advantage of it had the option been available, instead of maneuvering to avoid it). As explained above, when it does happen, it often is no issue either, altough of course, you are optimized for the ranged weapon, so a finesse melee weapon may do less damage. I think we had only one situation, when he was balancing on a statue suspended in mid air being attacked by flying demons, where it really was awkward.
I think missing this one feature, because is rarely comes to play, does not make your feat too weak compared to what the designers are considering as an updated version of the feat.