@Lord of the Large Pants I will respectfully disagree as well, even though I enjoy the perspective. In looking at the relationship however, I think there are a few things to break down, some you even point out.
FP... well considering Cinnamon Bun, I'm not quite sure what she was after, but still.
This was a large problem with the relationship and the main thing that set the stage for its later issues. FP had little of her own drive or character, simply existing to be Finn’s GF. She was cute and perfectly represents a passionate, fiery love, but not the long term stability either needed.
FP was just released from captivity prior to Finn. Her personality was largely structured around him as a means to get away from daddy flame. The biggest hinted conflict is whether she is evil or not, creating shaky grounds for a relationship, and her only definable desire being to not be controlled. Finn ended up breaking this and episodes like Vault of Bones definitely played a little into this prior to “that episode” in Finn trying to restructure FP.
FP is also highly insecure due to the daddy dilemma, which would further cause conflicts. Same can be said of Finn who got all sad boy because FP didn’t laugh at one joke, which means they are over.
The two are chaotic together, a giant ball of fiery passion. The destructive, passionate nature of both is the draw for the couple and the audience to the couple, but that mixed with the insecurity was going to bite them in the ass.
As far as the physical incompatibility... yes, but also no. Finn and FP both expressed a willingness to figure it out, and Ooo is full of all kinds of outlandish magic and crazy super science. I find it hard to believe they couldn't have come up with SOMETHING if they'd had more time.
The flames were symbolic as well as a real threat. Finn loved her passion and strength, but that passion isn’t everything. So much of the relationship was figuring out the physical, which adds to the incoming eruption of not getting each other beyond, let’s say what it was…sex. Finn liked the physical, the danger, with her but was not prepared to take on anything else.
I don't know that Finn did try to hero his way out of it? It's pretty strongly implied that when he tried to explain himself (which wouldn't justify it, but still), FP didn't really listen. She was just acting out of hurt. That's not necessarily unexpected when you feel betrayed I guess. It still would have made a lot more sense to me if she'd taken some time to think, and then given things another shot.
No, Finn was a red flag.
She wasn’t mad about the betrayal, or at least, only mad about that. That is the primary thing Finn did not and could not understand. What Finn did was take secrets and insecurities only he would know about and weaponize it for the purposes of getting her to do what he wants. You can blame the cosmic owl, or Jake, but the large extent to which Finn aimed for the heart with that letter can only be blamed on him. That is why she didn’t take him back, there really is no coming back from a love who clearly exploits your biggest vulnerabilities. He dug into her insecurity issues highlighted in previous episodes, along with becoming a further extension of her father in trying to control her.
This is also why his apology meant nothing till the later seasons. He didn’t get that the letter and its contents were the issue, not just him trying to set up matches with her and Ice King. Also should be highlighted how quick he is to try and move on and get back together, seeming more like a desperate move than a genuine apology.
Did FP mature faster? I don't know about that. She had major responsibility thrust upon her, but it seems to me like she didn't handle it that well.
She found herself and what she wanted, then took on the responsibilities of a leader. She moved on and was able to decipher the issues with the relationship way before Finn, even if her no lies and full honesty policy was extreme. Finn meanwhile needed two additional years and tons of character growth moments to understand why calling your girlfriend stinky and blaming her for not meeting your physical needs is a bad thing.
And sure, FP ended up focusing on being a ruler, but Finn could've fit into that. He wanted to be Bubblegum's knight, and that's a large part of what he ended up being, even as an adult. Just without anything romantic.
He really couldn’t, even beyond the maturity and vile things he said, he is too much of a wild card to be a knight. Even in F&C, the main cast admits they barely know where he is a good chunk of the time. He is an adventurer, he will do things like fight bad guys on a train for eternity. This isn’t reliable knight behavior, and why another wild card like HW, or even Marceline, seemed more suited to him when you move past the knight and Princess aesthetic.
Edit to add:
FP definitely started as a rebound from Bubblegum kicking Finn to the curb, but I didn't feel like it stayed that way.
Finn did shit like try to act mysterious so he could “keep his options open” while dating FP. He also would run and take bullets for PB like he did when Lemongrab attacked her. He never really got over her, he always acted as though he could switch up if things work out.
While I don't dislike Huntress Wizard, I ultimately would've preferred to see where they took things with FP. I feel like FP and Finn had a better dynamic.
The thing people like about HW is that it was Finn’s ultimate growth moment. He actually took the relationship slow and played on her terms, which was different to FP where it felt like her terms came after his. He also didn’t let Jake push things into getting physical quick, deciding the friends with potential romance was the better option. It just seemed healthier than the bombastic things he did previously.
Then with F&C it is clear they have a stable relationship that operates to accommodate the adventurous, wild nature of both. A happy medium after Finn played simp for PB and manipulator for FP, we finally get a relationship without any special drama, even if far less passionate and interesting.