I assume he isn't lying in that tomb, but he put himself there at the exact moment, to insert himself in the party as a move in his scheme against his heirs?
The post-credits scene heavily suggests this is just his first move in putting down those three after the absolute/no-souls things crossed some line?

maybe that body is just some random priest of his that he's inhabited for the purpose of this plot?
there's definetly something bigger going on with him than just, I was in a tomb until you woke me up, but more, the three I entrusted with my power have squandered it and have a plan in motion that will undo the very nature of divinity and where our powers come from, for this I am assembling a crack team of, hmmm what adventurers look like they are manipulable into stopping them, AHA look a nautiloid crash where the survivors have anti-absolute powers, a crack team of those guys and will aid them in taking you down!

I assume BG4 will be more Withers moving against the Three but this time, more directly or something as it's plot?

Minthara is the best character and she NEEDS to be recruitable if you side with the grove!
Also- I support the important thread in the suggestions: Let everyone in the Party Speak