Based on your setup though, the caster doesn't need to "dominate" the combat by taking on everything, since you've set up NPCs to be with them. Well built Rogue and Cleric NPCs could easily help take down the combat for either choice. For example, if the wizard paralyzes one monsters for a round or two, the rogue should obliterate it with critical sneak attack damage. While concentrating on whatever primary spell they want, said wizard could be dropping fireballs, cone of colds, or other non-concentration damage spells while the cleric protects & heals them. You're correct that no single "I win" spell is going to exist, but you don't need it with a party backing you. If you want to make it work, you have to run it with a single PC, which is going to be swingy in combat for both choices.I kinda want to see how the casters will dominate in this encounter by itself. Its a bit more involved since there are so many high-level monsters.
I want to challenge the very notion that a single combat encounter completely favors a caster. I have a sneaking feeling that these "single combat days" don't actually pay attention to the full adventuring day exp budget and only present fights that were basically easily winnable anyways. In this scenario, its a bit harder to cast just one spell and completely win, especially since concentration and multiple enemies may make lockdown spells trickier to pull off.
As for the adventuring day guidelines, a day's value is going to be Deadly+, but that doesn't necessarily mean anything either. I've put out Deadly x3 encounters before and watched my party brush it off like nothing (I've also had a Hard encounter almost TPK). A variety of monsters will be needed to figure out the full balance, and I truly doubt there are any monsters that are equally difficult for both.
Going back to the NPCs, a well built Cleric and Rogue can bypass these spending few/none of the PCs resources. If you want a proper challenge, you've got to leave these to the PC only.But I'm excited to be proven wrong if I can. I don't mind being wrong, I just mind being told I'm wrong but not having any confirming experience.
However, there will still be external encounters which run chances of exhausting their resources like traps and non-explicitly combat encounters, so I want to see if casters can reserve enough resources at the end of the day.
I want the full high-level one-encounter experience to truly feel what the community feels, I think I already know a 6-8, 2sr day will have significant resource expenditure.