I’m wondering if I could have something like :s but with JS regex syntax for instance, perhaps a plugin.
You definitely can have it, but you’d probably have to write it as a remote plugin in that host language. It would probably be a great “learn to write a plugin in a different language” project.
Lua patterns would be especially interesting here. Maybe that’s on the roadmap? They’re strictly less powerful than even POSIX regex, but that also makes them easier to write.
But after getting used to Vim regex, I’ve actually grown to like it as-is. The default backslash escapes are definitely annoying, but with \v and incsearch I have very little trouble writing complicated search and replace patterns.