You may like Project Oberon [1] designed by Niklaus Wirth [2] then. His guiding principle was to make a powerful but simple system that could be understood from top to bottom by a single person, from the RISC hardware to the OS to the compiler used to compile and run the OS.

It's quite a bit above the Apple ][ in terms of power.

[1] http://www.projectoberon.com

[2] https://en.wikipedia.org/wiki/Niklaus_Wirth