I think I've gotten to a point where I'm starting to grasp some mastery of the character.
For Mii Gunner, I mentioned that his largest weakness was that low damage from moves resulted in low damage racking and difficulty in getting kills. After another month of practice and a few tests vs. tournament-level humans, I was able to consistently land many combos, often times getting 15-30+ of safe damage from a variety of moves.
Nair has medium speed at ~frame 8, but it covers him and autocancels into attacks
Jab is fram 5 and some 3-4 frame attacks can hit before jab3, but it is easy to cancel the jab into other attacks such as utilt, dtilt, or another jab and you're always at frame advantage to block an attack. You can cancel jab1 and jab2.
Dthrow->nair combos at low percents and dthrow->uair combos at higher percents
Fair combos into the majority of his moveset. At low percents, fair-> dash attack, dsmash, and grab are my favorites. At mid % and above, you can combo 3-4 fairs into each other
For killing, the initial hit of the frame 5 utilt can kill around 120-130, which is pretty rad. A simple fair slide into utilt is pretty easy to do.
Facing center stage from the edge, frame 9 dsmash can kill at 90-100
Uair covers the full air dodge duration, making it a common kill at the top ~70-90. Uthrow is an easy setup if the blaster hits.
You can SDI Usmash, but it still kills around 110 and it's relatively fast at ~frame 11. It's really difficult to escape if the first hit lands against a grounded opponent.
Dtilt is pretty decent at high rage, too, when used at a range where utilt would miss and dsmash might get blocked.
Then of course, there's fsmash at the edge. It sucks for damage, but consistent above 100 at the edge and safe with a grenade setup. I find dsmash easier to land, though.
The above are without the grenade and missile specials. With them, you can apply insane pressure and safe damage. Missile deals moderate shield damage, bomb drop deals extreme shield damage, and grenade keeps you in shield. Bomb drop->dtilt can destroy a nearly full shield. Since you have full aerial movement with grenade and bomb drop while retaining fast fall speeds, smart use of regular B and B-reverse allows complete control over your arsenal. I found it easiest to mix between grenade and bomb for the best offense, with a mix of missile on the side. It is easy to combo a fair, grenade, or bomb drop out of other projectiles.
Offstage is where he arguably excels the most. You can send off grenades, bombs, or missiles and edgeguard offstage with fair or nair. UpB 2 can also be used as an incredibly quick spike offstage. You can go deep and return with a double jump + fair and it will kill if it hits. You can also choose to cover most ledge return options and relax at the edge.