What can Orcus do with Time Stop?

Last night I pitched the Demon Lord Orcus against my level 19 players. With a 26CR, I expected it to be a bigger challenge than it was. Orcus had a Wand of Orcus, and summoned a Nightcrawler, a Death Knight, and a Poltergeist against the 5 party members.

While holding the wand, Orcus can use an action to conjure undead creatures whose combined average hit points don’t exceed 500.

Our Necromancer stole the Nightcrawler, our Monk stunned the DK for 3 rounds, and the Poltergeist got the wrong end of our Paladin’s Holy Avenger. Anyway, as the fight starts, Orcus catches some of the players off-guard, and Time Stops in his first turn (no one was able to Counterspell it). But then… I had nothing to do during 5 turns. I moved Orcus closer to the enemies, and laid a Circle of Death, which ended Time-Stop. I’m not sure what Orcus should be doing when using Time Stop.

Initiative doesn’t run during Time Stop, so I couldn’t use lair actions to make more zombies.

Orcus causes up to six corpses within the lair to rise as skeletons, zombies, or ghouls. These undead obey his telepathic commands, which can reach anywhere in the lair.

All his undead-creating spells take 1 minute (Animate Dead, Create Undead). All his other spells or abilities affect creatures (aside from Detect Magic). Is Time Stop only useful to run away? Or to maybe force enemies to burn a higher-level counterspell? How can Orcus use high 1/day Time Stop spell offensively?