I hate to say it, but I feel like I'm kind of mainstream as far as prog metal goes; I really don't know if I'd consider anything in my list lesser known. I think my issue is that I have no idea how to find new music anymore (short of EveryNoiseAtOnce, which I just remembered existed).
The first album that came to mind, which is definitely not metal, is 100% Yes by Melt Yourself Down. I believe the genre description I read was "Afrolatin inspired post-bop punk-jazz", so of course I had to check it out.
I'm considering switching back to Spotify for just that reason. I jumped over to Tidal a few years back as they pay artists more, but their recommendation algorithms are garbage in comparison.