I'm not familiar with some of your applications, but I'm very familiar with dissembling and reassembling interiors and it sounds like others think an impact is overkill for you. For interior work I use a Bosch 12 volt pocket driver. It'll break fairly tight fasteners, and even without taking advantage of the clutch ring you can feather in a nylon fastener easily. I've used the M12 version also but prefer the Bosch, it's smaller, lighter and feels more precise to me.
I have an M18 Fuel as well, it'll generally come out only if I'm removing seats or seat belts. The Bosch 12 volt impact might be a good choice if you really feel you need impact. It's not available in brushless in North America yet but I can't imagine a day's work where you'd use more than a full 2AH battery. They also now have 2.5 AH battery in the same form factor, and a 4AH that adds some bulk to the bottom of the tool.
The pocket driver though, is available in brushless, it's the PS 22. I'd like one, but my PS 21 just keeps going and going. It's taken some significant falls and been used hard for years. It's a like a trusty old friend to me.
A few folks said you might consider square drive and they have a point. I'd say though, that if you're using bits even 20% of the time, just get a good selection of hex drive nut setters. I have VIM and Wiha in short, medium, long, hollow and magnetic and rarely use a socket, but I almost never need anything other than 7, 8, 10, and 12mm.
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