IMO the final fight has to be high stakes, hard, phased, and both success and failure need to have mixed consequences, rather than all or nothing. Maybe they beat the BBEG, but lose a beloved NPC. Maybe they claim the Ring of Wish, but it is all out of charges. Maybe they TPK, but arise in chains in some underworld, where they are shown their failure with a glimmer of hope: though they failed, their efforts weakened their foe enough that someone else stepped in to finish them off.

Afterwards, “win” or “lose”, there should be no happily ever after, only a dissolution of the status quo: the future is open ended, but it won’t be the PCs—one way or another—who do the work of building it. Their saga is over, through some form of ascension or destruction, and thosr PCs are no longer available for play: they might, if they survived, cameo in a future campaign, or become NPCs, or even the next BBEGs.

You want to strike a fine line: there must be closure, but it can’t all wrap up neatly. Your players should feel catharsis that this show is over, while making them yearn for a spinoff. I recommend NaDDpod as a live play with incredible examples of this at the end of each of their (now many) campaigns.