Hiya, Nice start. You are crazy indeed, but get stuck in and make it, then find what fails and improve it till the whole lot eventually works! That's half the fun.

For a bar spinner like that, 500mm is in my opinion too big, the robot will be huge and in the robochallenge arena at least, will barely be able to get up to speed + will throw itself out immediately if it ever does. Decimators was about 350mm effective diameter for reference

The aluminium section on the front is way too weak sorry, that will crumple the first time it hits something 25x25x1.6 or 2mm thick steel would do though, very cheap too.

The NTM is fine to get you going with the aforementioned mods for reliability as long as the weapon sits on efficient bearings. They have decent enough torque especially the super low Kv ones like that. Check out the SK3 motors too, they are substantially better made and tend to have more power IMHO.

Probably have way too much battery as well. Lots of weight can be saved by using 1x for the entire lot, or at least a way smaller battery on the drive. 4s 2000-2200mah is plenty, the drive will be fast especially on 125mm wheels (they might be too big btw, probably will break the gearboxes) and you shouldn't burn out motors. Or go for a bigger 6s pack and run the weapon and drive off it.

Absolutely go for the TZ85s over the 30As on drills, trust me on that one.

Good luck Keep us posted