MM Phono cartridge

Yes, that may be the right path to pursue.  I also wonder if upgrading my Origin Silver tone arm ( which is essentially a Rega tonearm) would be beneficial or " better spent" money.

I wonder if anyone has found tone arm upgrades to be an efficient deployment of audio dollars?

How about considering a HO moving coil, a Hana SH MkII, for example. If your desire for an MM cart is to avoid the additional preamp stage, a HOMC does that. If you have some other characteristic I'd love to know what that is. 

Consider this review: https://musicalsurroundings-my.sharepoint.com/personal/ken_musicalsurroundings_com/_layouts/15/onedrive.aspx?id=%2Fpersonal%2Fken%5Fmusicalsurroundings%5Fcom%2FDocuments%2FA%5FPrice%20Sheets%2F2024%20Price%20Sheets%2FHana%2FHana%20Reviews%2FSTRP%5F4711%5FDREAMS%2Epdf&parent=%2Fpersonal%2Fken%5Fmusicalsurroundings%5Fcom%2FDocuments%2FA%5FPrice%20Sheets%2F2024%20Price%20Sheets%2FHana%2FHana%20Reviews&ga=1

The Hana SH MkII is $850, so you can keep $1000 extra in your wallet, confident you haven't left any performance on the table. 

Either Nagaoka or Clearaudio are good choices.  In your budget, you could probably do better buying a Denon DL-103 and sending it off to Andy Kim or another re-tipper for an upgraded cantilever and better stylus and adding an inexpensive step up transformer.  That would be more dynamic and musical than most MM carts.  Not sure how the Denon would work in terms of compliance with your arm.