That is disappointing about the 500W motor.

Do you have a test gage to test the run out on the inside of the collet holder?

I recently bought this nice little set:

https://www.ebay.com/itm/333761931981

I had to use a piece of steel flat bar stock for the magnet since the bed is aluminum.

Which replacement controller do you have in mind? I am happy with the GRBL controller except for the noisy speed controller.

Depending on your power supply, you may need this "PWM to Control Voltage" converter:

https://www.amazon.com/dp/B0BFLC7YMB

I think you would have to figure out how to direct wire it to you power supply.

I purchased this for another machine I have for a Mach 3 controller to GRBL board conversion.

I have 3 other large mills and will just use my 3018 for wood instead of upgrading. I bought it just to learn GRBL to convert my other machines to CNC.