A bunch of ideas have been running through my head on next steps but many involve a 3.3V processor, which is sometimes TTL compatible with 5V, but sadly, according the MAX7219 data sheet:

Logic High Input Voltage VIH 3.5V

Bummer.

Sure, I can make little step-up circuits for each of three data lines, or get a quad level converted module, but either just means more wiring, complexity and expense.

While there's a 3.3V "equivalent" of the MAX7219 (the MAX6951), there's no pre-made or semi-made LED matrix modules available with it. And after skimming the DS on it I'm not convinced it'll even run an 8x8 LED matrix.