Been down this rabbit hole before, and honestly, the pickings are super slim from the original animated canon. The 1997 film kind of leaves their 'happily ever after' as a finished piece. Disney did publish a few young reader chapter books in the '90s, like 'Hercules and the Hero's Heart,' but those are very much kids' adventures retelling the movie, not deeper dives into their relationship. You might find some fan theories floating around about whether Meg's past or her bargain with Hades gets more play, but officially? Not really.
Where you do find a ton of expansion, though, is in the wider world of Greek myth retellings. If you're craving a complex take on a 'Hercules' figure and his relationships, Madeline Miller's 'Circe' has a fantastic chapter from Circe's perspective about the hero she calls 'Aristaeus'—it's a brutal, humanizing portrait that contrasts sharply with the Disney version. For Megara specifically, Jennifer Saint's 'Atalanta' touches on other heroines and their often tragic fates, which gives context to what a 'real' Megara's life might have entailed. It's a different vibe entirely, less musical comedy and more poignant tragedy.
So, no direct sequels, but the mythological genre right now is rich with material that explores similar themes of heroism, trauma, and love with that epic, mortal-cost feel.