Hi,
I think those DC spindles are not up to the task.
They are just DC motors with deep-groove ballbearings and a ER tool holder clamped on the end. They have crazy high runout.
You need a spindle that has angular contact bearings. Angular contact bearings need to occur in matched pairs. So a low power spindles, say 800W,
will have 'four bearings' arranged in 'two pairs'.
I have been making PCB for eight years and with just one spindle....its been absolutely superb:
https://www.mechatron-gmbh.de/en/
It is a German made spindle, and so is not cheap, but its proven its worth many MANY times over.
Craig