[ardenum2]

Would getting rid if the C frame be enough to host a proper BTR30 spindle with a servo?
I will check the dental mills.
If I were to use BT30 driven by a servo instead of a router ATC that use ISO30, what additional benefits do I get? Such as tapering threads?

Regarding power of the servo, I see that there is no much of a difference in price, so for 3 axis I guess 600W or 750W is also ok. But if the price gap is substantial, I would like to stick with 400W if that's possible.

for a bt30+servo a c frame is probably the best but needs to be beefy to take advantage of it. If you go down this route high speed machining is probably off the table, these bt30 cartridges usually run at max 12000rpm, most cap out at 6000rpm due to the bearings, which is why you need more stiffness to support heavier cuts.

vulcanmachineco(attached) makes a belt driven bt30 but not an a c frame, its a double column raising gantry, the stiffest we in the diy can hope to ever get, but to replicate it in EG could be tricky, you'd probably have to redesign the saddle instead of resizing it and figure out how to fit bt30+servo onto it.