Honestly i thought Santhids used their tentacles passively rather than actively, since they're just hanging in the water below they could catch a lot of plankton. Also if they're poisonous Santhids could catch some fish with them, while also functioniong as a defensive measure to predators. With the hard shell and (presumably) good vision and sentience, the santhids wouldn't be anythings prey as they can defend themselves so well with a hard shell, and the only soft part being coated in poison, just like some species of jellyfish.
Also, I've wondered if Chasmfiends evolved from Santhids, presuming Rosharan life, as ours, began in the ocean. Santhids moving to shallow waters, or the waters becoming shallower, would lead to them developing shorter tentacles as the extra length would be worthless.
As the Santhids moved (or got moved) closer to shore, their tentacles would be used for pushing along the bottom in shallower places, and their main food source would turn from plankton, to fish as the volume of water they could sweep got smaller, which caused them to develop a mouth to better ingest fish and the like. Some of the tentacles would then evolve into claws to catch and manipulate prey. As the tentacles got stronger to push along the bottom faster, and even shorter, they eventually got covered in shell to make them more rigid for easier movement.
This would lead to santhids developing into amphibians as they have now essentially developed legs. As they are now on land they developed smaller eyes which are easier to shield against highstorms. On land they they developed to what we now call Chasmfiends.
I don't know if Santhids have gemhearts but I don't think it's too far fetched to believe they do. What a Santhid does with stormlight I don't know. They could potentially be using it to gain knowledge through the cognitive realm and visiting Shadesmar, which would partly explain their sentience.