If you don't want to start completely from scratch, may I recommend you take a look at the VideoBrain? https://en.wikipedia.org/wiki/VideoBrain_Family_Computer (only language ever available for it was APL/S -- "structured" APL.)
Also, the "Decker" Hypercard-alike has a K-inspired language that is pretty nice. (Yes, much more modern, but still feels a bit like something "of that era".)
Or you could just start with ASCII... I was discussing how shaders work with a friend and wound up hacking together a sort of "shadertoy" that runs in Emacs last night. Scroll to end of file to see examples...
You may be already aware with this, but if not, Walker actually did write a small Forth (ATLAST -- https://www.fourmilab.ch/atlast/) which was used for (I think) DXFTOOL (a file converter).
It was later used by a small computer graphics studio in Tennessee for a blue/green screen matting utility and a fast roto-paint program.
A very long time ago, sometime during the first geologic age, I worked at a facility on Queen Street in Toronto. On the street side of the building, we had two Flame suites (very high-end (for the time) realtime editing and effects, used for composing television commercials). Each one had a Sony Trinitron TV of about this size as the client preview monitor. They were amazing, but every time a streetcar passed outside, they would get involuntarily degaussed!
Also, the "Decker" Hypercard-alike has a K-inspired language that is pretty nice. (Yes, much more modern, but still feels a bit like something "of that era".)