The torque a gear can handle greatly depends on the surface area of the contact patch, so a wider 1.5 Mod gear could handle the same torque as a narrower Mod 2 gear.

Also depends on the hardness of the gear material, the pinion gear usually is case hardened.

My first robot used Sinclair C5 motors that used a 5:1 Planetary gearbox with plastic gears. Was only 250W at 24V but I ran them at 36V and had no issues with the box.

I wouldn't use belts (personal preference), its the heavy, expensive, and doesn't allow for misalignment caused by being smashed by spinners! If space allows use chains, if tight on space, use gears.