AT 33SA or OC9 for 1200 mk5

Ultimately, your ears will determine what is "best"
I've used the OC9III. Great cart for the price, if your referring to that model, before the many iteration of new OC9  that were introduced last year. The OC9II is even little less$ and purportedly "better" than the III. Either way, for the price it's a good value.
https://www.lpgear.com/Merchant2/merchant.mvc?Screen=SRCH

I haven't heard/read anything on the newer OC9's. Likely all good, for their price point.

If you can up your game a few more, the ART9 is unquestionably a step above. Read the threads.I've used one off an on the past 5 years. Presently on my table.

Save your obsession/audio nervosa regarding compliance. Just get an AT whatever and enjoy the ride.

i would opt for the AT VM540 MM cart with your table. then sell the Phonomena, and look for a used Chinook. The VM540 cart is over looked because of its lower price. but its a musical cart, and mates extreeeemely well with your table and a chinook phono stage.

"If you can up your game a few more, the ART9 is unquestionably a step above"

I’ve heard that from a couple people... I’m also a believer in "buy once cry once" also so that doesn’t help my budget...lol.

Surely is an option but have to do a lot more homework first.

Hana LS!

Well, i’ve seen some people using even 32g SPU cartridge on Technics tonearm with a custom made superheavy ring added to the counterweight, but in my opinion this is a perversion. It’s like mounting a wagon on top of your sport car.

Technics tonearm designed for MM cartridges, check for the mass of this arm, it's relatively light mass, that was a 500$ turntable back then, if you never re-wired your Tonearm it is a bad idea to use MC cartridges on it. The weak part is some sort of a "motherboard" where the tonearm wires goes to, look here. It’s crazy that there is a weak connection between 5 thin wires from the tonearm and external rca phono cable. You do better get rid of this strange connection, you’d better solder some proper tonearm wires (like Cardas or Discovery) directly to the external phono cable (i like Zu AUdio Mission Phono) on this board, to avoid signal degradation. You have to use cardas soldering with silver.

A low output MC cartridge signal is very sensitive to cables, shielding and everything before the phono stage.

This problem is irrelevant for MM/MI cartridges.

The old Technics SL1200 mk5 is nowhere near the new SL1200GR, the tonearm on the old Techics is not as good as the new one.

For cartridges like ART-9 you definitely need a better tonearm, it is easy to replace the arm on your mk5.

BUT if you want to stick with the stock tonearm try to find some killer MM or MI cartridge like Grace, Victor, Audio-Technica, Grado, SoundSmith, Garrott, ADC TRX II, Stanton/Pickering ... There is nothing to lose for you in your setup between MM and MC.

BTW: Saec SS-300 mat is great for this deck! Isonoe footers are great, you can even buy KAB Fluid Damper for high compliance MM. Along with full rewire you will upgrade your mk5.

It is much better than to buy a LOMC and notice hum and phonostage problems later on.