Here’s one you might not have seen before: pilot, which comes gratis with any recent installation of alpine.
If you use alpine (or re-alpine, as the case may be), you probably recognize this as the file selector tool that pops up when you attach a file to an e-mail.
pilot reminds me a lot of fm, mostly because it bounces quickly between directories, and mostly dumps files into $PAGER.
pilot does have some basic file manipulation tools though — copy, delete, and so forth.
As you can see, pilot does a good job making use of available space. That vimwiki directory probably has about 450 files in it, and pilot shows a lot more of them at once than Midnight Commander does.
So for a light, fast, file viewer that looks familiar and meshes nicely with your environment, pilot is a pro.