Gyrodec Best Tonearm

If you can find an Alphason HR100 mcs would be a great fit for your turntable.

If the Zeta is working well, keep it. Change for change sake is expensive and unproductive.

I prefered Zeta on the Linn LP 12 above FR-64 mainly because of weight. Though Zeta is also a 'heavy gun' .
This 'factor' is related to the used springs. On Thorens
160 Super the Alphason HR 100 was a better match.
Your own idea to send the arm to Johny of Origami is the best of all propositions. Your problem is the cart not the
tonearm. The Ortofon Kontrapunkt C or Jubilee are much
better. If you intend to experiment with MM carts then
the Alphason is the best choice. On ebay the Zeta is slightly more expensive. I assume of course that the money
issue is important.

Regards,

I'm currently using an Ortofon Cadenza Black cartridge and it sounds wonderful on an Ortofon AS212S tonearm, not quite so good on an SME IV tonearm with an uncomfortable peaky treble. This is mounted on an SME 20/2 turntable.

Instead of upgrading your tonearm, have you considered upgrading the Gyrodec with updated power supply/bearing/platter ? You might get a bigger return from power supply upgrades and or an overhaul of the Zeta by Audio Origami - bearing check and rewire.