Probably what you want to check out is Oberon, which is a modern PC built basically from scratch, along with an operating system that compares to Windows, Linux, or MacOS, built originally not by a single person but by maybe a dozen people. It's capable enough that it was the daily driver for numerous students during the 80s; the earliest versions of it were built in-house by necessity because graphical workstations weren't a product you could buy yet. Wirth's RISC CPU architecture avoids all the braindamage in things like the Z80 and the 80386. I think that, with their example to work from, a single person could build such a thing.

Oscar Toledo G. also wrote a similar graphical operating system in the 01990s and early 02000s, working on the computers his family designed and built (though using off-the-shelf CPUs). You can see a screenshot of the browser at http://www.biyubi.com/art30.html and read some of his reflections on the C compiler he wrote for the Transputer in his recent blog post at https://nanochess.org/transputer_operating_system.html.

There's a lacuna in the recursivity of Wirth's system: although he provides synthesizable source code for the processor (in Verilog, I think) there's no logic synthesis software in Oberon so that you can rebuild the FPGA configuration. Instead you have to use, IIRC, Xilinx's software, which won't even run under Oberon. Since then, though, Claire Wolf has written yosys, so the situation is improving on that front.

CP/M is interesting because it's close to being the smallest system where self-hosted development is bearable; the 8080 is just powerful enough that you can write a usable assembler and WYSIWYG text editor for it. But I don't think that makes it a good example to follow. We saw this weekend that Olof Kindgren's SeRV implementation of RISC-V can be squoze into 5900 transistors (in one-atom-thick molybdenum disulfide, no less) https://arstechnica.com/science/2025/04/researchers-build-a-... https://news.ycombinator.com/item?id=43621378 which is about equivalent to the 8080 and less than the Z80. And Graham Smecher's "Minimax" https://github.com/gsmecher/minimax is only two or three times the size of SeRV and over an order of magnitude faster.

There's no reason to repeat the mistakes Intel made in the 01970s today. We know how to do better!