How much RAM does the computer have? I am thinking it cannot be a lot for 1970's tech.
21-bits could do RGB777 colors.

The system is designed around serial I/O and core memory emulation. It just does not have the speed
to do any kind of bit mapped graphics. 4.71 Mhz ( 4x clock) . I only have a 19 bit address space- 256K words of RAM lower
memory ,ROM and I/O in the upper memory. Memory is 32K words typical. I have a hidden 32K of memory
for stack and I/O buffers.

In the early 70's you only had B&W bit mapped graphics.
Xerox Alto 1973 had 96K ram and 606 × 808 pixel (portrait) display , keyboard , disk and rodent.
DG Nova instruction set. A 20 bit computer design just needs a another 4 bit, bit slice, so ignoring
timing issues, I could have system with bit mapped graphics, but that beyond my skill level in design.
This is a job for Super FPGA
Ben.

I was thinking along the line of the TV typewriter or AppleII graphics, destined for a composite monitor or TV. Text mode may not use much memory.
I thought chip memory like 2102's were being used, but maybe that is too advanced. for 1973 ish.

With 21-bit wide memory, 7 3-bit pixels could be read in a single cycle lowering memory access requirements. The dot clock could be 7x the 1.x base clock rate.


FPGA would make things much easier, but much less authentic.

The notion of a circuit being implementation in 1970's logic.

What is the motherboard bus like?

This is more a hardware emulation, than a true retro design. I am cheating in few things like using a 6850 ACIA and a SD card
but I feel the design has the spirt of a TTL computer. The 21 bit computer is a after thought for for the working 20 bit computer
as it can be hacked into the 20 bit cards.

I disagree, with the right FPGA macros, with schematic entry the data path is well covered.
micro code ROM as the issue, as different venders do things different.
Four phase computers? had 24 bit alu and did it that way about 1975.They were leased rather than bought
so they where not well known. They also had their own custom memory and logic chips.

A 6502 or 6800 style bus for the 6850 ACIA. Right now it is at the get it working stage.
I have a serial terminal for use when a get a stand alone system, so a video display.
using old VGA monitors, because they free was not considered with early 70's time frame.
Also they need a 25 Mhz dot clock, so the bus would need to be revised. Color and inverse video also need
to addressed, so 20 bit machine might be better since you have 8 bits for each character, and 2 bits for color.

To be honest video is the place to 'cheat'. Video with TTL isn't worth the effort in my opinion. It's a lot of chips to do very little very quickly. Chips and board space better spent on making the CPU more interesting. The exception is monochrome(or two colours) with shift registers. You get readable text and usable graphics plus you can pick a nice colour that won't burn you eyes from being oversaturated.

Instead, you could emulate an era appropriate graphics system or tty with a cheap microcontroller like the Pi Pico or Esp32. Have it monitor memory writes and shadow an area of RAM that can be used for graphics data or text. FPGAs are an option as well.