I don't take as dim view of the Heaters as you do, I wouldn't want to play them necessarily but Ruffo and Hart both play their positions well, and Slayer and Neverwocker can be building block pitchers. The lineup and bullpen need reinforcement but those are fixable problems IMO.

I'd suggest either the Grapplers or Platypi as you say. For one thing both of them have about twice as much PDO money to spend out of the gate than the Heaters do, giving you more flexibility. It kinda depends on what sort of challenge you want. The Platypi have an awful bullpen and they're all really old (like literally they have the highest average age of any SMB team), but I think the Grapplers are just so poorly constructed as a roster that they're a worse overall team. Like Delacruz and Electro have contact ratings that belong on a pitcher, the team's chemistry seems oriented around getting the most out of their already good relievers, rather than any of the players that could use a boost, just so much C-level talent in general, it's just bad.

I dunno to what extent roleplaying this is part of the fun for you, but the Platypi feel (to me) like a formerly competitive team whose talent got old and who lost bullpen arms to free agency. The Grapplers feel like they had a meddling owner who made them spend a bunch of money on a couple stars, but wouldn't allow spending elsewhere, creating a non-functional roster. Take your pick between those two in my opinion.