Looking good so far, just some idle thoughts π€ (and you would have to mount the gearbox back maybe 50mm) but you could slip one of these in series to lower the gear ratio?
https://www.injora.com/products/metal-1-3-ratio-reducer-planetary-gearbox-transmission-box-for-axial-scx10?_pos=8&_sid=73fd91a17&_ss=r
https://www.elitemodelsonline.co.uk/Cars/By-Manufacturer/Lesu/Axles-Suspension-Drivetrain/94480-/Lesu-114-Planetary-Reduction-Gearbox-141
I have a 14:1 for my next project waiting, only I have started building something else π I will add I found the Carson motors variable (even though the truck world swear by them) and I used a Holmes 540 crawler motor in place, very tractable, they are all satin black too.