If you could use some of the Tuxcart resources/engine, perhaps a vehicle-based battle game with a garage for customization of your vehicle?

I’d say keep it simple to start. Perhaps a turn-based tactics game like final fantasy tactics advance, or more 3D, like XCOM?

I haven’t tried it out yet, but the cypher system seems like a good fit.

Just a suggestion for a free web-based VTT, owlbear rodeo is super simple to learn, and quite flexible.

I haven’t tried it out on mobile, but I’d bet it’s worth a try.

https://www.owlbear.rodeo/