opam 2.2's release cycle has fallen a bit behind the compiler's (actually because of the Windows support). It's an experimental branch, but this works with opam-repository-mingw to get a vanilla mingw-w64 build of OCaml 5.0.0:
I saw some kind of other installer that might come with either MSYS2 or Cygwin I forget which, but it also said that installer takes 2 hours to run. They seem to strongly recommend WSL over it based on how I read things.
Not for 5.0, the aim of 5.0 was to focus on getting multicore support out of the door. Thus only mingw64 is supported for Windows. Support for MSVC will come with later versions. At the same time, improving opam ecosystem support for Windows is one of the major goal of opam 2.2 . Thus hopefully the support for native Windows will improve in the future (next year?).
I now run mainly on windows and this is an issue for me to try OCaml