My buildSystem is currently Ubuntu, so I can't just flip the nixos switch for binfmt emulation. I could probably apply the binfmt config manually if I want to go the emulation route, right?

Yup binfmt is possible to setup on all distros

Ideally I could use the cache and cross compile only if needed, but it looks like I can't do that without manually choosing specific packages to be cross compiled with pkgsCross attributes in the first method, right? And if I did want this, then my buildSystem would need to support emulation, right?

Can't automatically pick the packages to get from the cache and which to cross-compile. You can however make a second native nixpkgs instance to pick packages from

Ideally I could use the cache and cross compile only if needed, but it looks like I can't do that without manually choosing specific packages to be cross compiled with pkgsCross attributes in the first method, right? And if I did want this, then my buildSystem would need to support emulation, right?

Can't automatically pick the packages to get from the cache and which to cross-compile. You can however make a second native nixpkgs instance to pick packages from, you could even overlay them.

Can't automatically pick the packages to get from the cache and which to cross-compile.

Could I introspect the cache somehow (outside of nix, ahead of time) and add a nixpkgs overlay based on pname to select between ( native | cross ), hoping that the hashes will then match?

You can however make a second native nixpkgs instance to pick packages from, you could even overlay them.

If I do set up binfmt emulation, and I'm using a stable nixos branch that hydra has cached, and not building custom packages, should I expect a 99% cache hit rate, and then the last bit is essentially making the sdcard images which I'd need to do locally?