Cartridge IS the most important.
IF your phono sounds great; IF you can hear differences between cartridges, then I would not even consider changing the phono stage until you change the preamp and/or amp.
I would consider these changes in the future:
1. Turntable with Two Tonearms, (If into Jazz: Mono ready to go, back to Stereo in seconds in a listening session). One arm easy height adjustment and changeable headshell. Other, perhaps longer arm, perhaps fixed cartridge. Consider method of choosing which Tonearm.
2. Low Output MC Cartridge, which means: new SUT to go to existing MM Phono; or New MM/MC Phono Stage with SUT built-in. Have PASS to send MM cartridge thru the SUT/Phono Stage unchanged. Consider MC SUT (internal or external) optional xFactors and optional impedances for future cartridges.
3. Tubes! Thus if new Phono Stage: Tubes, separate, or in tube preamp, or tube integrated with Phono or you go separate tube phono.
4. Speakers: to easily try tubes, high sensitivity/efficient speakers are important. Existing? New?.
Less needed power decreases cost/weight/heat; increases competition in that power category, increases placement options