You might just want an FPGA at that point. It won't be competitive with modern CPUs (by an order of magnitude), but it will be fairly well-documented and more flexible than most other boards. You could target RISC-V with emulated cores, or try your hand at implementing custom logic.

Raw performance isn't much interesting to me: I'm not planning on running production Postgres on it, just to prove to myself that computers are orders of magnitude nimbler than what experience in our day-to-day. Windows 95 had less latency on a 486 than most desktop OSes on a Ryzen 7. The sheer amount of layered abstractions we keep piling on and on isn't helping.

In any case, I don't know anything about FPGA, any board to recommend, that's useful for a pure software engineer with only a passing knowledge of electronics?

mepian

Look up the boards that use the Lattice ECP5 chips, I think these are the largest FPGAs that have a fully open toolchain: https://github.com/YosysHQ/nextpnr

You'd need to learn some digital design for any FPGA board. I recommend the book "Digital Design and Computer Architecture" by David and Sarah Harris.