Then best stick with Xamarin. There's still nothing better for F# dev on Mac currently (and unfortunately).
Xamarin have some very smart people on the case.
Limited code-completion, but Atom has an F# plugin, you could also just use any plain old text editor. This is what I have been doing, personally, and I just use FAKE for builds.
Yeah, Ionide is pretty good. I personally use vim-fsharp because Atom is flaky and VSCode doesn't have a good vim emulator yet, and vim-fsharp does the trick well enough for most of my usages.
Apropos of nothing, I recognize you from #ubuntu-offtopic on Freenode. Hiya!