[WesM]

Linear encoders would be my choice even with the Gecko drives. There is no downside to using them, and many advantages. You just have to match the resolution to what the Gecko drive is capable of reading.

Yea I'm researching that right now. The Centroid requires a minimum of at least 1000 ppr encoders and recommends 2000ppr/8000cpr encoders. Trying to find documentation on the Gecko to see if it supports that.

I initially purchased the CUI BLDC encoder development system for programming the simulated hall tracks for the BLDC motor encoder version, but got anything but satisfactory results, and the tech support was no help, so it left a bad taste.
I traditionally used Renco, but then Heidenhain took them over and the line selection shrank quite a bit.
Koyo are good as well as Hengstler.
If possible I usually fit through hole type to avoid mounting and a coupler.
Your original servo's may have had tach feedback which is not used now on DC motors, as Torque mode drives are used as opposed to the DC velocity type which had a tach back to the drive.
Al.

I will look into those brands shortly. They may very well be tach feedback. From what I have read/seen, replacing the old resolver/encoder and leaving the tach in place but not hooked up should be ok?