Accurate Strong Gantry

[vargai]

Why don't you sketch your idea for the spindle and investigate the bearings.

Here is some sketch and I am almost sure the for DIY use with a short 5 mm flute cutter this spindle is quite strong enough.
Cutter is clamped by Weldon @6000 it works, lower AC bearings are coupled and after installation it will be preloded
grease lubricant also fine here.
Some safety issue should be considered:
The drive shaft in the ER20 collet should we carefully machined including the pulley to reduce the unbalance.
I would use double key opposite to each other.
Good cover must be done to protect the belt and user.I checked my 170 mm blade Makita plane , it run @15000rpm so you are of course right regarding the belt application. Here ribbed V belt is used so i think 15 mm wide HTD3 is sufficient here.
If you flip the pulley on the mini spindle to the top to skip the belt from the milling area you have to use some adapter plate .
This is not the first thing i make now but probably later Paris is well worth a mass.