I want to address one accuracy error source - misalignment of router bit and ring axis. Here is my take on that challenge - forcing both into align position.
I design a “frame” which hold router and legs of ring in proper position:
And printed it in PLA - now legs are perpendicular to sled, ring is forced into rigid position and router (in my case it is AEG) sits exactly in center;
Here is full assembly;
And two closeups with details
If you like it let me know how upload stl file.
Stay tuned for test runs, but first I am rebuilding my table.