It might be cheaper (or at least comparable) to ship it to Elektron and have them install new encoders (if they still do off-warranty work on the MKI) or have someone local do it than to buy a Beatstep. It’s a pretty straightforward repair so the labor costs shouldn’t be too high.
I use an external controller for track volume on my MKI and it works really well, but personally I’d want to get the OT back in full working order before I added anymore hardware.