ceramics are really bad for any place where it can chatter. they are very brittle and can't recommend it for spindle.
I am doing a similiar thing. instead of servo, I scored some brushless bicycle motor (400watt) that has built in controller and was affordable. weighs 5lb and runs 2800rpm at 24v. I am planning on keeping the belt drive (and by adding a 4" pulley, I want to hit 13000rpm) with my broken in spindle (which has been running 10k for over 6month and runs just slightly warm at 10k), I think I can get it up to 13000 without it heating up too bad. generally, by replacing the grease with oil, you can usually gain 2-3k rpm anyway, but require regular oiling)
if you want to make your own higher speed spindle, you can get just the er16 spindle with 1/2" diameter, which would allow you to run 17k bearings(abec5) you will need different spindle body though. I think I can be happy with 12-3k with variable speed and keeping the 6 speed belt drive.