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Have you thought of using vehicles from WW2 gaming?
A lot of Epic scale vehicles had their 1:600 equivalents done as fan projects (IIRC printable in 3d), and there are 15mm eqivalents.
It's certainly not a bad idea ... in fact, it's pretty good. But how to give them a little sci-fi "oomph"? (Weird camo patterns? 40K-style liveries?)
The other consideration with WWII kits is that they are available in a lot of different scales, and you can alter things by eg using a 1/72 vehicle alongside 15mm (if you want it to be large) or alongside 28mm if you want a small vehicle (or even a large droid?).