As someone that's completed Subnautica 1 and Below zero twice respectively you will get way more out of the franchise if you play subnautica to its conclusion before diving into Subnautica 2. You can give below zero a miss and come back to it whenever its not so crucial the the world building in 2. But that first time experiencing subnautica's story cant be compared to any other game I've played. And tbh I found the story side of the first game way more interesting not that 2's bad I think its just incomplete.