Why not just play a few of these in your Doomsday decks? Maybe even 4? It’s a fine spell on its own.
Against Dredge, Shops, and DPS a resolved Doomsday is GG. Thassa makes the win slightly less conditional. (it opens up a couple of rare situations in which you wouldn’t be able to win with Lab Man) Playing more Thassa doesn’t really accomplish anything. It would be like playing 4 Tendrils in a DPS deck or 4 Blightsteel in a Tinker deck.
Doomsday struggles against decks like BUG and Xerox, which is why it’s really missing 4 Gush. You can’t play good enough card advantage spells to keep up with Planeswalkers. By using Planeswalkers or something like Arcanist to play the card advantage game, you’re having to sacrifice Doomsday components and go back to losing to Dredge/DPS/Shops for not much improvement against Xerox. Playing 4 Thassa in this situation would actually make you worse against all decks since it’s just a 1/3 with a scry ability.
By playing Doomsday you’re not playing the game WotC wants you to play and you’re bound to lose to the power creeped cards that aren’t applicable to your strategy.
Doomsday had been a deck that was good for its ability to battle toe to toe with other tier 1 blue decks. It just doesn’t have the draw power anymore to keep up. You can start with Ritual Necro or Ancestral, but the odds aren’t in favor. I’ve played Doomsday in numerous online and paper (NYSE) events in 2019. It feels like it has potential but it’s an uphill battle.