You're right on that first point, and I believe they've already done that. Varmints had Marceline helping PB get through her control issues, and in subsequent episodes we see Marcy making a concerted effort to help her relax more, see: Broke His Crown and Ketchup. Stakes had PB helping Marcy sort through a lot of her unresolved trauma and mature as a person.
I think they are at a place now where a relationship between them can work, and it would be beneficial to both of them. The only reason why it would be 'too drastic' is because they haven’t made it explicit enough. I wish we could see how they navigate specifically being in a romantic relationship because of the baggage they both have, and for the fact that it is an odd pairing and it would be interesting to see what exactly attracts them to each other. I understand the reasons why we haven’t gotten this and probably won't, but still.
Calling it half-assed is unfair in my opinion though. They put a lot of effort into developing their relationship since WWM. Take the romance out of the equation and they've done a great job.