Not, It's sleeping, on Demiplane, creating his army e regaining resources. It is very rare, very rare for the Sorcerer to need to return to his Demiplane.
You explained that the sorcerer was on a demiplane as a counter to any scrying attempt. The Wizard 20 can scry at you thrice a day, more if he upcast the spell, and somehow he would always do it just at the time the sorcerer is on the demi-plane... and yet this is supposed to be "very rare"? If it's very rare, the scrying worked and the wizard has seen your fortress, temple and sanctum (though not the inside, just the outside). Plus, as you're "sleeping" inside, I guess it would be a daily occurrence to regain your 6th level slots and above as those can't be created through sorcery points. So instead of being rare, it should be "daily".
But as the Holed Up Hovering Target is already losing, having forgone all semblence of social life just out of fear of a Wizard 20 who doesn't even know his name, I'll grant you that and petition the DM to be lenient AGAIN on your behalf. So the HUHT is resting, rarely, when he needs it, on his demiplane. OK. Cool. I concede that he Wizard 20 can't scry on him there.
I notice that Demiplane is on his spell list, which is cool for the sorcerer. But the spell can get the sorcerer to a demi-plane, but not get him out of demi-plane: by RAW, and your DM is adhering to RAW, Demiplane opens a shadowy door to a demiplane (either a random one or a previously encountered one). If you don't cross back during the hour the door is open, people are trapped in the demi-plane. Which is usually not a problem for a user of magic, since anyone worth its salt can planeshift back to the prime. Except the sorcerer, who his spending his daily wish on Private Sanctum and can't emulate the 7th level Planeshift spell.
Logically, the routine must be: he is operating on the prime material plane usually, shrouded by his Private Sanctum. At some point, one day, even rarely, he decides he needs to sleep, so he cast Demiplane, using his 8th level slot. So far, so good. But, after he wakes up, he has regained the use of his 9th level slot but... he his trapped on his demiplane. His only way out is the planeshift back to the Prime, which has two drawbacks. First, he has planeshifted back with 8 creatures at most, so his horde of magen will likely stay. I guess he will take the simulacrum preferentially, but feel free to state if it's 8 magens. Second, planeshift doesn't allow you to specify an exact destination: trying to get to the City of Brass can get you inside, or looking at it on the other side of the Sea of Fire. It's the canonical example given in the spell description. Since "DM please help" is outlawed by the rule of the thread, there will be many times when the HUHT will be thousands of kilometers away from his fortress, temple, sanctum when he gets back from his demiplane. On one of those occasion, the Wizard 20, who is trying to scry daily on the basis of his second-hand information, will succeed at scrying you and notice you are in a vulnerable position, with at most 8 creatures and in the open (and, but he can't know that, your 9th level spell slot gone).
By introducing the Demiplane into the Holed Up, Hovering, Absent-minded Target, you just created several days of opportunity to strike for the wizard, without your retinue of creature, and at least one day without Private Sanctum or any other 9th level spell. It is on this day that the Wizard will strike, I propose, since for the first time his scrying is actually working (and the wizard doesn't know how long it will be before such an opportunity arises) and the Sorcerer appears to be ill-prepared. Unless you stipulate additional details on the routine of the HUHAT (which would affect the wizard strategy), I think we could consider this occurrence, the first the scrying spell actually worked, as Day 1 of the Wizard 20 attack and start discussing opening strategies.
What are the 8 creatures the Sorcerer take with him when he planeshifts back into the open?
