I started 3 years later than you on a 4K Radio Shack computer running a 6809 8-bit CPU. I can still remember quite a few opcodes and a few months ago helped some on retrocomputing.stackexchange.com find a rather tricky 6809 assembly language bug without looking anything up. It's weird how stuff sticks around in our heads, since I haven't written any 6809 code in more than 40 years.

Despite being entirely self-taught due to never having programming classes or books, I enjoyed a long, quite successful career as a serial startup entrepreneur. In 1981 the only info for 'toy micros' was small hobby zines, local user's groups and just learning by disassembling other people's code. I started with the ROMs in the computer and learned by looking up the opcodes on a Motorola quick reference card. I got it by cold calling Motorola trying to sound like an 'adult' and the salesperson took pity on me and mailed the card for free.

I still think there's something invaluable about learning computers from first principles down to the metal.

May I suggest you check out the game "Turing Complete" https://turingcomplete.game/

It sounds like it may be right up your alley. Its a simplified digital logic simulator, you make basic circuits from NAND gates, and then build these into functional units in later levels, then further on you build an instruction decoder, and combine it with these functional units and before long you have a Turing complete architecture and you are writing your own assembly on it - Assembly language that you defined, from scratch. You'll find the need for new opcodes... so, you build them! Challenging game for sure, but very, very rewarding.

The game is still in early access, but very near a full release. They just put out a trailer: https://www.youtube.com/watch?v=goclUECM2ds

Both the Z80 and the 6502 were products of corporate rejection.

I'd love to see another designer introduce a FORK86-64, but things are much more complex now.

Same! Also built external peripherals for my ZX81.