You can pretty much pick up Summoning from any album, they started out metal-focused with their earlier releases like Minas Morgul, so those might not match the “chill” vibe. That said, everything after Dol Guldur starts to lean all the way into Dungeon Synth. Personally, I think the best album to start with is probably Let Mortal Heroes Sing Your Fame, I’ve always felt it was the moment they perfected their sound.

The lowest hanging fruit for recommending here is probably Summoning, that’s about as close to a band who’s entire concept is “Chill Mordor Ambiance”. Alternatively, anything in the Atmospheric-Black Metal to Dungeon Synth pipeline, Ornatorpet, Paysage D’Hiver, or Old Sorcery. You might not find the screechy vocals to your taste, but it has unmatched Atmosphere.