>Nautilus, in my opinion, is no longer a file manager. Rather it is a content discovery tool.
Completely agree.
>Open to suggestions (just trying to avoid one of the bajillion naughty-less-forks)
Caja seems pretty full featured, but I'm not sure how tightly integrated it is to MATE. The only other FM I can think of that might be able to be integrated into Budgie without pulling in a buttload of dependencies for some other DE, would be the XFE File Browser (no relation to Xfce or Thunar).
IMO, Caja is the only good FM available for Solus, but it'd be nice to have a good one that's independent of DE.
I think the XFE File Manager or even SpaceFM would be good alternatives since they're DE agnostic, but unfortunately they were both rejected in the Phabricator a while back. :(
Also @ /u/Pandasticus
X File explorer should do at least the bulk of what you're after, and if there's features it doesn't have that you need, I have no doubt there will be some stand alone apps available.
Check your distro's repository, but here is the website: http://roland65.free.fr/xfe/
Everybody wants different things in their file manager. You should certainly try a few of the suggestions here. Usually there is very little effort to install them and get rid of them if you don't like them. XFE isn't tied to any DE, so it works on about any distro.
It is a bug reported by a Xfce developer months ago and not fixed. The Xfe file manager is more modern, silent and has more features.
It is available at least in Debian.
I suggest you to give a look (and perhaps a try) to xfe GUI FM if you haven't already, I find it more powerful and easier to use than ROX, and has lighter dependencies ;)
PS: Appreciating the choice of Sylpheed