~
I once dove into the assembly space with a MIPS processor, and took great joy in talking directly to the CPU.... that was until I learned that assembly itself is not a machine language but an *interpreted* one, because newer CPUs now perform their own optimisations in the hardware space!
After that I grew listless and world weary, and I closed the curtains in the basement and listened to depressing 80s music.
I marvel at your patience and gusto.
How about a Commander X16 or MSX3? The MEGA65 is too expensive.