You're 100% correct. It's also possible to jump it up further with the items found in Tasha's. I don't want to mutate this into another TCE thread however, so I'm just going to say "I don't really like them" and leave it at that.Wouldn't the save DC be 19 (or higher, if you have one of those DC boosting items from Tasha's)?
I agree. He can summon a bunch of liches instead of dracoliches who could probably Dimension Door him out and use Counterspells to protect him from high level spells, but this is something that will probably fly over most DMs' heads when put on the spot.8 + 6 proficiency + 5 Intelligence = 19, at least.
Even so, he'll probably save most of the time, but even that isn't necessarily a problem for a Wizard with Simulacrum, True Polymorph, Forcecage and all that. Orcus doesn't have a teleport in his statblock from what I can tell, so he'd have to rely on a summon to get him out of a Forcecage, definitely an issue if he's caught unaware.
You don't enjoy beating on blobs of meat?Garbage monster design is another major issue with 5e that deserves its own thread.