I have only played Subnautica 1, I haven't managed to get Below Zero, yet.
Wasn't the first game just crutching forward? They had tons of issues in the development process, I believe it was another of the early access trash. Literally the only thing the game ever had has been its unique theme.
I remember reading they wanted randomly generated worlds, but they concluded to the static map. Was that really a design choice or a technical limitation, i.e. they couldn't get things to work? When the game released I think it had mid performance at best, and it had stuff missing. Hell, the game still has an "unstuck" option in the pause menu. There are mods to unload Aurora, because, apparently, the shipwreck is always loaded, regardless of where you are on the map (PirateSoftware approves?). Still if you spin the camera around you will get a nice spike on your fans.
Nothing I have seen or vaguely remember has shown that this was a team of extreme competency. They are good, but that is all. Many quality of life things were added after the release.
And then Below Zero. The DLC that became standalone... The main complaint I remember is the size. And maybe some retconing, yeah. But I do not have any real opinion for that, not having played it yet.
To their credit, when Steam Deck came out they rushed to make the game verified. That is the latest update of Subnautica 1 on Steam.
So, how would Subnautica 2 be any better? If people didn't love the rather unique theme of the game, Subnautica 1 would have probably never been finished. And they were just lucky to be competent, in contrast to many other games.
From the "gameplay teaser" it feel like they got all the good components of the (first?) game and improved them, made them... cooler. Maybe even better. But separately. They might or might not make a good result when put together.