It didn't work.
Instead I migrated to nix + python directly.
I keep the way to use autocommand to set the formatting.
But no expression but an external program.
This program is created via nix as shell application.
This one uses the sqlparse, just like the plugin.
This will use the std input as file for input.
That way I can re use this in other places.
Integration is way easier, no additional overhead to write python
embedded as vimscript.
Also full dependency control via nix.
No need for overlays.
Instead create own packages and call them via callPackage.
That will keep them simpler and allows dependency injection.
I can follow existing patterns from nixpkgs, etc.
Daniel Siepmann