And we're back to trying to guess what you are doing. Since the spell doesn't seem to be among the known spell and Private Sanctum takes 10 minutes to cast, I suppose we must guess that Lazy is Wishing for it? Right? So he's casting that on the next round (since he already cast Subtle Dispel on the preceding round. So, since he didn't do anything when the Wizard cast his spell, sometimes it will have been something not useful, but sometimes (when facing a high-level wizard...) it could be Teleport or Planeshift... so when on the next round Lazy gets to casting his Private Sanctum, he'll already be far away.
Except if he's already far away, having taken the rational step of moving away when surrounded by an unknown number of unknown opponents with magical capabilities. He didn't wait for your to cast another spell. If really he had, for some reason, cast another spell after your Subtle Dispel, he'd be inside the Private Sanctum... he'd really realize he'd need to move away, wouldn't he? He could still cast Demiplane depending on his DM's interpretation of what consists of planar travel (demiplane not being a plane, it's left the individual cosmogony of the world). Maybe his last round spell was a lowly Haste and not some strategic moving capability and he could just Move and take two Dash action to run 180 ft away. The is a strong possibility your own Private Sanctum is now obscuring view between you and the fledgling Wizard.... What do you do? [So far, most high level Wizards will have escaped...]
Nobody can be surprised without DM fiat. Even if it was, the surprise would have prevented action when you cast Dispel, not on the subsquent rounds.
You have trouble counting up to three? I understand why you'd want to ban the use of magic item, probably a bad experience handling the Holy Handgrenade of Antioch?
Shocking Grasp has a range of Touch (you could Distant it to extend range to 30 ft) and Polymorph is 60 ft, potentially extended to 120. The Hasted wizard is 180-300 ft away depending on how far you were when you cast Dispel.
On his round, the Hasted Wizard Dashes twice and Move, to get in a 360-480 ft range, unless he's high level enough to teleport away, since he'd be outside of the Private Sanctum range. You're near to, well, let a Wizard whose only spell cast was Haste (3rd level) escape.
You're right (about being a good evil boss). In fact if the PCs in my campaign reach high-level, I'd like to make them face a "bad PR campaign" led a villain, because at endgame it's too easy to deal with "usual" problem (the ones with HP).