I imagine it would be easier for me to build the simulator than to make it do anything of interest.

While snarky, I see where you're coming from. Sometimes a lot of the fun is in the building of a thing to understand how it works. Learning how a thing works from the user-facing side of the control panel is, for some of us, less rewarding.

If anything though, I want to go build a physical relay computer mechanism myself now (if even just a 4-bit adder or something).

Ha, I really get the sentiment! I do recommend perusing the Book 1 manual [0], towards the end there are a number of small problems (on page 30). They're worth tinkering with :-)

[0] https://minivac.greg.technology/manuals/1961-minivac601-book...