to answer some of those:
skill feats are less important than general feats as far as "combat effectiveness" goes. You also get a lot less general feats than skill feats.
So with those in mind, Quick repair you should definately buy with a skill feat and not a general feat. Keep your general feats for the more... general stuff, like toughness, fleet, improved initiative, ancestral paragon, and etc.
as for when to get it, it's nice having it early on, even from level 1 (but not necessary), because it takes 10 minutes elsewise to repair, which is the standard amount of most things you do in-between combat (so as an example someone may refocus, which takes 10 minutes, or do some treat wounds, that takes 10 minutes, and etc) BUT it also takes you 10 minutes to recharge your Unstable if you've used it in that combat. So, now if you also have to repair your construct, it would mean waiting for 20 minutes. While with quick repair it means that instead of a 10min break, you do 11min breaks, which is more or less the same.
gadget specialist is "neat" but i would say it is better for melee inventors, due to blast boots being the best gadget (imo always) and that is mostly a mobility tool. I'd take it mostly for flavor, so if there are other important upgrades, like your construct feats, i'd certainly prioritize those.
Mega/giga volt is very nice ability, mostly when you get the gigavolt upgrade it starts to shine, because i have never been in an occasion that i couldn't ping-pong it off enough surfaces to get all the nasties while avoiding all the friendlies. One of the best aoes for martials.
But that's it, an aoe. If you are picking it only for the ranged option, you already are using a bow. Plus, you can only do 1 unstable per combat in most combats, and you should already be picking up the repair one for your construct. paired alongside the explode that you start for free with, that's already 2 options to spend said unstable.
Construct companions in general are a bit more sturdy but also a bit less accurate than regular companions, so they serve better as meat shields, especially since you have an unstable to repair it in-combat, so i'd prioritize using it as such instead of trying to use it in range. Especially if you go wizard, that also gives you even more ranged options, like a round of cantrip+bow" is servicable enough.
That said, 3 feat slots, in teh grant scheme of things, are not a ton, so you definately will have space for more stuff to put in.
Speaking of wizard and buffs, companions do not benefit from item bonuses except to AC and speed, so runic body is useless to them. Enlarge will work fine though. That said, it's not like an archetype will ever have enough spell slots to properly "support" although as a wizard, if you fill your spellbook with miscellneous exploration spells, you can bring those to help, if you have the full day to switch from your normal spell selection to those. But it won't be a combat thing.
An alternate Int based spellcaster that you can try is psychic, especially if you go with picking up Guidance as your amp, since you normally don't have focus points as an inventor, and that will give you yet another renewable resource to spend in a combat. I said Guidance because you are lacking a good reaction as a ranged inventor and you said you wanted to support, but you could go for an offensive amp as well if you want to double down on that. keep in mind though, that your dc will be behind a caster.
