Now, having already 2 things that really ought to use 5V I decided to split the system: the 5V Mini Pro takes care of the keyboard and text-LCD, talks to the 3.3V Teensy via standard UART serial which uses the only voltage level converter.
I also implemented a 40x30 virtual display that can be scrolled using the keyboard, without the Teensy knowing about it. Really useful!

Following this I added an SPI TFT display directly to the Teensy. I had used this display on my Raspberry PI at 80MHz, so I knew it was good for at least that speed.
After getting it working at default 20MHz speed, I tried to raise the speed but garbage above 56MHz. Of course, I had breadboarded the display with long wires to the Teensy. A change to much shorter and soldered wires meant I could run at more than 96MHz :D
I've implemented 6502 memory addresses to access functions in the display, and writing simple basic-code to unit-test.