It's too bad I missed the cultural milieu of the classic Macs, so I lack the motivation to go down this particular road with you. But I'm on a nearby track with other simpler (and non-retro) computing stacks.
I personally have enjoyed some toying with 9front and Uxn, and am always interested in others.
https://github.com/akkartik/mu
https://love2d.org (e.g. http://akkartik.name/lines.html)
