Hey everyone,
For what it's worth, the game successfully compiles and runs on 10.14 and probably earlier (had a bit of a weird time with the 'simutrans' folder inside the git repo, but once I sorted things out it worked). I'm not very familiar with how GitHub actions set up their environment, but you should be able to specify an earlier version by setting the env variable "MACOSX_DEPLOYMENT_TARGET" to something like 10.12. It ought to compile and at least try to start on earlier OSes, and I'd expect it works.
It's also generally a safer bet to link libraries statically on macOS, as you've surely noticed using install_name_tool is annoying. The trouble here is that Apple's linker always picks a dynamic library if it finds it (and I do mean always, no matter what settings you use), and homebrew generally loads dynamic libraries as well as static ones. So you'll end up with the dynamic version.
On 0 A.D. we worked around this by building libs manually, but this is quite cumbersome. A very ugly hack is to simply hide the .dylib versions when compiling. You can also keep bundling them with the app.
Cheers anyways.