Below Zero was originally planned as a DLC for Subnautica. It was also made by a different team. But it ended up being larger than planned, so they made it a full game, intending it to be Subnatutica 1.5, the same as Miles Morales. Unfortunately, they communicated this poorly to most players, and they took it to be Subnautica 2, which made it underwhelming. For some reason I took it as intended and liked it well enough, and I only found out it was disliked when I went online and saw people calling it Subnautica 2. They seem to have won, because Subnautica 2 has been rebranded as Subnautica 3. But I still call it Subnautica 2 because I remember the history.
As for the seatruck, I loved it. I never liked the Cyclops. Sure, it was impressive, but it was also a giant trashcan with a “kick me” sign on it. I was too terrified to drive it down the lost river, I preferred my Prawn suit with its actual mobility and comparative stealth. The Seatruck was a mobile base that was actually practical to use. And I loved the customisability.