Excellent job.

Fwiw..
My lathe has similar resolution with 10.000 count encoders, driven at 1;2.'I went with 8 mm profile, 30 mm wide, HTD belts, and taperlock bushings.
Reason:
Smaller belts are not stiff at all, and cannot transmit a lot of torque at low rpms, under say 10 rpm.

8 mm profile HTD is about 2x as stiff as 5 mm, and 30 mm is obviously twice as stiff as 15 mm wide.
Even then, I wish they were more rigid.

Another thing to take into account-
For good stiffness, belts want to be quite tightly tensioned.
Look this up, and make sure You don´t over-tension the shafts on the motors.

Maybe You already knew all that, just pointing it out in case it is useful.

Good luck.