I still have an MC20 in a drawer, which also has that insanely low 0.07mV output. I remember a long time ago, I enjoyed this cartridge when I had a combination of extremely high gain preamp > 20dB) and power amp gain (> 30dB) to make up for it downstream. Very different sound from modern Ortofon. I sometimes consider how I would handle that cartridge today.
First, you can upgrade your MM gain above 40dB, as you mention. The ARC Reference series phono stages (Ref 3) are pricey but get you a whopping 52 dB in low gain / MM mode on the balanced outputs (6dB less in SE). Many other tube phono stages offer 42 - 46dB, like various VACs (some built-in to preamps), Rogue Ares, Hagerman Cornet / Trumpet. The Herron VTPH-2 is a good one, especially if made with the 4x 12AX7, 1x AT7 configuration which gets up into 48dB MM (drops to 43dB with the 3x 12AT7, 2x AX7 configuration - note a unit’s RIAA curve will be factory tuned to one config or the other; you shouldn’t just swap tubes). I believe EAR 834P also has a very high MM gain.
Then I’ve also been considering - what about using a low ratio SUT in combination with a moderate gain MC stage? It sounds "wrong" but on paper there’s no reason it can’t work great. For example:
EAR MC-4’s 40 ohm tap (10x = 20dB gain) plus a Benz Lukasheck PP-1 62dB (all solid state gain) results in a whopping 82dB gain, should be very low noise, and 220 ohm net load. You can use RCA splitters and loading plus if you want to lower the load further. OR how about a nice low noise hybrid MC stage like Sonic Frontiers Phono 1 (came in 54dB and 64dB variants) or Manley Chinook (60 or 66dB) should also work well like this. Their lead stage is JFET so the loading is flexible. You definitely CAN’T stack two SUTs in series (because of loading and Ohm’s law) and you shouldn’t stack two MC gain stages (because of noise) but a SUT into a JFET MC stage is fine (kind of seems like an audiophile “life hack” here), as long as you set the input loading above 1K!
So - lots of options!