Oh. My. Gog. That is some serious work.
That truck vaguely reminds me of something ... hmm, what could it be? =))
https://rctruckandconstruction.com/s...ad.php?t=13841
But nowhere near the level of detail you're doing. With the interior, I don't know how you have room to squeeze in a power train, even @ 1:10.
I feel your pain on flexible filament, and IMO you're crazy (in a good way!) to 3D print so much. Especially tires, when they're available off-the-shelf.
Wrt fasteners, I've been using the brass threaded inserts. You print a hole just slightly smaller than the OD of the insert, as determined empirically for your printer and material and brand and color and phase of the moon(*), and then either draw the insert in mechanically, or heat it and insert it with a soldering iron.
(*) Buddy of mine describes 3D printing as "an iterative process." Possibly the best way I've ever heard to say "Trial and error. So much error." =))
But again, OP, hats off to you. I spent a bazillion hours on mine so I can't imagine what yours will entail, but it's a beautiful thing.
-- A