Nice looking design!
Some advice specific to belt in tube:
The biggest difficulty with belt in tube is assembly and maintenance. You will want access holes on the top of the tube above the pulleys. Even still, getting the shafts to line up with the pulleys and bearings requires some finesse. We are planning to move to Thunderhex which will hopefully help. You can check out the CAD for our 2019 drivetrain here. Our access holes have tabs that we bend up slightly to provide a smooth edge for the belt.
Once you have the belts inside the tubes, your entire team needs to be disciplined about keeping debris out. If metal shavings, rivet heads, or other things get in, your belt will get shredded. Make some polycarb covers for the access holes. We let our guard down this year, and a shaft collar ended up inside which shredded a belt. Fortunately this happened during practice at our shop.
I don’t have any experience with 3D-printed drive pulleys, so I can’t say how well they would hold up. The failure mode would likely be rounding of the hex bore. 30 tooth pulleys will be fine. We use 24 tooth, but have not tried it with anything larger that 6 inch wheels. We buy aluminum pulley stock from BB Manufacturing, which we machine and hex broach. You can find our pulley CAD here.