The pinnacle of MM is 70s/80s production, not sure if you’re familiar with this
MM thread with a lot of discoveries. With your budget you can buy great MM or MI cartridge for sure, the Glanz (Mitachi Corporation) for example patented MF (Moving Flux) generator, must be used with MM phono stage.
You already have Moster Cable LOMC (designed by Nakatsuka-San, now ZYX). People who prefers LOMC are happy about this cartridge, but you’re not so happy. The phono stage for LOMC is much more critical than for MM, also cartridge loading etc.
I think a decent MM or MI is what you have to try, at least $500 is not a big deal, but it must be a right choice of cartridge.
I’ve linked the TAS article about cartridge neutrality, one of my favorite is Audio-Technica AT-ML170 and AT-ML180, both are much more expensive than your budget, but Stanton or Pickering with Stereohedron stylus is cheaper than your budget. Or maybe lower models in AT-ML series like the AT-ML150 with MicroLine stylus. These cartridge are neutral.
I’ve never used SoundSmith cartridges, he made all B&O cartridges back in the day and repaired nearly all cartridges in existence. I think we can trust his words regarding MI even if the lecture is just a promotion of his own brand.
However, only you can judge a cartridge in your system.
In today world every $500 cartridge considered "cheap", buying those gems from the 80s we often pay less to get much better quality and it’s true with MM design. For some exceptional MM cartridge made in the 80s the budget must be $1.5k today.
P.S. Brand new $12 000 Top Wings cartridge is MM, designed by Hiromu Meguro, former design assistant of the Grace F-8 cartridge and principal designer of F-9 cartridge. I love Grace and collect them, got some uber rare Grace models and they are shockingly good, but $12 000 for a Top Wings is a good illustration of the modern High-End world and its prices.