Adv:
Forget the linears.

Use timing belts like HTD3 or GT3 with a big head and a small end, like 9:1 ratio.
Someone can cnc drill you a large circular head for little money.
From a hydraulic pipe end or similar, in a production machine shop.

50.000 count encoders are not expensive, and 2M counts seem around a few hundred €.
The best cnc cut bit belt end will still have errors in linearity and accuracy.
So what.
Software map them out, because the repeatability is 100x or more better than the accuracy.
All based on what I have seen others do.


*You* cannot make the system *accurate* to a high level.
It will bend and flex if changing direction.
But You *can* most definitely get extremely accurate incremental positioning for this use.
0.01 - 0.001 degrees angular increments are certainly possible - depending on your mechanicals.

Your system could get 100x more accurate in angular resolution via double crossed geartrains, preloaded, error-mapped, using ac servos with feedback.
Some potential private systems could make it fast in tracking.