“ThemeDir=” must also be adapted. But there is no option for that.
No, that’s the nixos system theme dir. Installing a theme package should put its files there. This is how we handle things like themes in NixOS: we point the program at a system directory, and then populate that with appropriate files from installed system packages.
The problem is that your theme package doesn’t put its files where it’s expected to (so that we realise that it has “appropriate files”). It needs to put them under $out/share/sddm/themes. Here is an example from my config.