Depends on how progressive you want to go, as when you are bordering on noise, ambient and other fringe genres, there are lots of bands with little fame.

I have a single album from “Lento” that is pretty cool. Post-rock, powt hardcore, post metal kinds stuff.

Nero Di Marte is on the more extreme side, but flirts with prog.

Oranssi Pazuzu is prog and black metal together. Weird stuff, but good.

And a long time favourite: Circle. Lots of different kinds of albums, most pretty great. Awesome if you can catch them live.

Edit. More stuff and typos

There’s this noiserock/prog group called Fushitsusha. The songs are long, winding and sometimes grinding romps. They have some traditional Japanese elements mixed in as well.

If you don’t know about Mono, go have a listen. Especiall Hymn to the Immortal Wind. Stuff brings tears to my eyes.

Not sure how popular this is but The World Is Quiet Here got a whole lot of eartime from me with their last album Zon.

Not lesser known particularly, but definitely (imo) don’t get the recognition they deserve: Wilderun

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.

Familiar with King’s X?