Rebuild or new cartridge with my zeta tonearm

My only concern on the existing Sumiko is that it may just be a tad light for the Zeta.
It was originally about $850 new, not sure if I would spend $500 to $600 on a rebuild.
It will likely not have the same boron cantilever on return.
Now it is entirely possible that the OP may really like the sound of the rebuilt cart but is an expensive gamble IMHO.

Lyra Delos

tzh, the Delos is well over birdfan's budget. Birdfan, All of my cartridges save one are LOMC. I do believe, actually I know you would be better off with a MM cartridges than an inexpensive MC cartridge. The Clearaudio Performer V2 fits the bill perfectly. It is $450 and is a good match for your arm. It tracks at 80 uM which is excellent. It will be more dynamic than any LOMC in your price range. Warning, stay away from vintage MM cartridges. They are all way too compliant for your arm. The Grados and High output Soundsmiths are borderline. You could use a low output Soundsmith  but they are about your price range.

Sounds Good? Perhaps preferred by some, however, the specs of separation and balance of the Clearaudio Performer are not good, they must limit imaging compared to OP's Sumiko.

https://clearaudio.de/en/products/cartridges-mm-v2.php

A refurbished Decca (or new - a little expensive) loves the Zeta arm.