Turns out that was, in fact, the BBEG, and mechanically, we just... JUST managed to win.
By the end of the day, it became very clear that... This campaign of more than half a decade is in the end game now.
We were only lv12. We're now lv14.
It was a 575k XP encounter, more than 20x the 'Deadly' rating for our party, and more than 8x the 'Deadly' rating for a 5 member party of lv20 PCs.
The DM straight-up said to me in a dm this evening that we're pretty much done, mechanically - anything he would put down to challenge us would take so long to fight that it just wouldn't be worth the time. He's still got some fun encounters to try out, and we've still got plenty of narrative to resolve, but we're kinda... We're definitely past the point of no return.
I dunno if I feel good about it or not; this might be the first time I've ever felt bad about 'breaking' a system, but also, the most narratively justified I've ever been in doing so? The focus now is building a satisfying narrative with everyone out to the end of the campaign. As long as everyone is happy with how it all goes, we're all good.
It's a legendary end to a legendary campaign, and, hopefully, the last 5e campaign any of us will ever play.