I don’t think there are any issues with using refind as a boot manager directly in theory. Something like this abandoned PR would need to be written though: [WIP] nixos/refind: init by betaboon · Pull Request #58121 · NixOS/nixpkgs · GitHub
That’s admittedly easier, just set boot.loader.timeout to 0. Technically there will still be multiple entries if you hold space, but that should be fine for your purposes?
If you want to write another entry with other settings, you can use boot.loader.systemd-boot.extraEntries for that.