There's an extremely relevant planet with a pretty much identical biome to Ahch-To in Legends. It's also one of only about five locations in the entire galaxy that was a rich source of cortosis. Perhaps the ONLY one that still had significant amounts during the waning days of the Republic. Bal'demnic-
Bal'demnic was an ocean planet adorned with rocky tropical islands in the Bak'rofsen system of the Auril sector, located within the Outer Rim Territories. The hostility of its native Kon'me species, together with its extremely harsh climate, discouraged most people from ever visiting the planet...
starwars.fandom.com
It was also majorly important in the Tenebrous and Plagueis stories. They both sought to control the cortosis resources of the planet. But it's also the planet where Plagueis killed Tenebrous and assumed the title of sith master.