Phono preamp worth it with MM cartridges?

I agree with the cart upgrade first. Try a Soundsmith Carmen Mk2. You can find the older body ones on sale for a very reasonable price for what they achieve sonically. Will be dramatic upgrade from the cart you have now. The Rega Phono stage is good for sure. But get the cart changed first. Then see from there...

MC cartridges are extremely low signal output (less than 1 mv) cartridges and therefore need very expensive front end phono preamplifiers. Inherently, their signal to noise ratios are usually worse than MM (3 - 6mv) cartridges due to this reason (-70db or so vs -90 db typically)

These days, a $500 - $700 MM cartridge with a very inexpensive but excellent Schiit Audio Mani ($150) phono preamp would be almost indistinguishable to MOST ears. Plus, you can replace an MM stylus but not an MC stylus, which needs expensive re-tipping.

Get a Nagaoka MP500 and/or an Ortofon 2M Black and you will be more than happy with the sound AND the money that you have saved.

Do not fall down the black hole of very expensive esoteric cartridges to gain another 5% of better sound, especially if you are over 40. Your ears will most likely not hear it.

If you insist on the absolute best phono preamps however, Pass Labs Xs phono ($45k)and/or Avid audio Pulsare ($8k) cant be beat.

When to take the step to MC? I would say you would get the most bang for buck by staying with MM. Get the 2M black (or bronze which you can upgrade to black for little money in the future) and partner it with the schiit mani. The mani is amazing sound for the price. That combo will sound so much better on your Rega turntable than your current set up. I loved that combination on a similar priced project deck a few years ago. I only moved to MC when I could afford to upgrade to rega P8, apheta 3 and rega aria. Aria is great but haven't tried it on MM. Am keeping the mani if I ever get the chance to have a second deck with MM. 

Yeah, all you need for LOMC is sill a decent MM phono stage and external SUT to match cartridge impedance. I’ve been using this pair of SUTs and it can cover all types of LOMC (from low to high impedance).

MM stage and SUT is a classic combo.

A great MM phono stage is a must have, in my opinion great MM stage should not cost more than $1200 (many available withing this price tag).

If you have a friendly dealer take the $2k and your P3, and exchange for a P6 with the Ania MC + Sutherland KC Vibe phonostage. THAT will be a very noticeable upgrade to your record collection.