Mythic Trait triggers only at 0 hp
Game features do only what they say they do. It is difficult to damage Rogues, between their Uncanny Dodge against attacks and Evasion against area effects. But magic missile, for example, will do full damage to them because it is neither an attack nor an area effect. Neither of the rogue's protective features will trigger against magic missile.
Similarly, Mythic Trait explicitly triggers when the Mythic creature is at 0 hp, only. Being killed or destroyed is not the same thing as being at 0 hp and neither will trigger Mythic Trait.
A creature can die without having been reduced to 0 hp first, if a feature says that it outright kills the creature. See Does dying cause a creature explicitly to drop to zero hit points? (link provided by Kyromaani). As argued in the answer there by fortyCakes, if dying was the same thing as being reduced to 0 hp, there would be no reason for the druid's Wild Shape ability to end when "you drop to 0 hit points, or die". This phrase is not redundant because dying does not put you at 0 hp and either situation will independently trigger the end of Wild Shape. The same wording is used for the end condition of the spells animal shapes, polymorph, and true polymorph. Similarly, a feature that causes the immediate death of a Mythic creature (such as your suggestion of Power Word Kill) will not trigger the Mythic Trait because the death occurs when it is at a positive hp total, and once dead the former creature has neither a hp total as a creature nor the Mythic Trait.
As far as the meaning of 'destroy', the word is used 56 times in the PHB, typically with its natural English meaning. However, it is used as a game mechanic in the cleric's Destroy Undead class feature (PHB p. 59). No DM I know has the undead that are destroyed by a cleric become 0 hp and start making death saves - rather, 'destroyed' bypasses their hp total and imposes a state change.
Thus, it is possible to kill or destroy a Mythic creature without triggering Mythic trait so long as their death or destruction is immediate and does not have to pass through 0 hp first. In this case, the Mythic Trait will never trigger. What follows are some suggestions for how you might do that.
Mace to da Face
Both Maces you cite have similar phrasing:
If [the target] has 25 hit points or fewer after taking this damage, it is destroyed.
Note that it does not say that 'if the target has 25 hit points or fewer after taking this damage, it is reduced to 0 hp'. Presumably the target is still at its potentially positive hp total when it is destroyed. As you suspect, this would bypass any effects that trigger at 0 hp. A mythic construct or mythic undead hit by one of these maces could, if between 1 and 25 hp, be destroyed without triggering their Mythic Trait. (Your mace might take the creature from more than 25 hp to fewer than 1 hp in one blow. In that case, it is a separate question whether the destruction or the Mythic Trait would trigger first, and if the latter, whether that would protect the creature from destruction.)
Thomas Markov notes that, at the time of writing, there are no Mythic constructs and only one Mythic undead, so these these maces may be of little utility as is. However, if you can true polymorph a non-construct, non-undead Mythic creature into the appropriate type for your mace they could be destroyed without triggering the end of the true polymorph at 0 hp (I would suggest doing the construct if that mace is available, since it does not get a Wisdom save against the Smiting). However, this creates its own timing question. It is clear that dying does not set them to 0 hp. But it is not clear whether being destroyed also counts as dying. Thus, if 'destroying' the true polymorphed creature means that it simultaneously dies, and since that would end its polymorphed state, would that then negate its destruction as it was no longer a creature type that the mace could affect? That is a separate question as well.
How tired is it?
Consider the spell sickening radiance, where:
When a creature moves into the spell’s area for the first time on a turn or starts its turn there, that creature must succeed on a Constitution saving throw or take 4d10 radiant damage, and it suffers one level of exhaustion
If you can force the Mythic creature to remain in the Area of Effect of this spell (or cast multiple copies of it), accumulating six levels of exhaustion results in instant death, irrespective of current hp total. A good list of other effects that can cause exhaustion can be found here, although most of them would be difficult to weaponize against your Mythic foe. Finding some way to have the creature contract cackle fever or sewer plague while it was in the sickening radiance might be the most straightforward of your options.
How small is it?
If the Mythic creature is smaller than 2' in diameter, or if you can get it there with reduce, polymorph, and/or other effects, you might be able to pop it into a sphere of annihilation:
The sphere obliterates all matter it passes through and all matter that passes through it.
Anything that doesn't entirely fit within the sphere has a save to avoid damage, and takes force damage if it brushes against the sphere. But anything smaller than the sphere itself is, by my reading, 'obliterated', which to me means death / destruction regardless of hp. YMMV, since it could be argued that the creature gets a Dex save regardless of size and so the sphere can only be used to damage, not obliterate, creatures.
No, really, how small is it?
Can you pop it into a bag of devouring, preferably after it has used up all its legendary actions but before its turn? If so:
Any creature that starts its turn inside the bag is devoured, its body destroyed.
How small does it need to be to fit inside the bag? Interesting question, because unlike a bag of holding, this bag does not have its dimensions specified. Further:
When part of a living creature is placed in the bag, as happens when someone reaches inside it, there is a 50 percent chance that the creature is pulled inside the bag.
Note that there is no size limit specified, so presumably if you can trick or coerce a Gargantuan mythic creature into just reaching inside the bag you have a chance of devouring it, hp notwithstanding. No mechanism is given for using the bag offensively, so it is not clear how big the mouth of the bag is for when you try to pop it over someone's head or other body part, or what sort of attack roll or contested check that might entail.