Holy crap I finally found out, and I don't need a mapping.

The magic search keyword was

    vi repeat last substitution

which yielded as first result
https://stackoverflow.com/questions/382155/how-to-repeat-a-command-with-substitution-in-vim#:~:text=You%20simply%20hit%20the%20.,hit%20.%20to%20do%20it%20again.

So `&` is what I wanted.

Apologies.