How hard is to dispel spell that got permanent by casting it every day for a year?

Mordenkainen’s Private Sanctum, Teleportation Circle, and possibly other spells have text like:

You can create a permanent teleportation circle by casting this spell in the same location every day for one year.

The Dispel Magic has

The DC is 10 + the spell’s level.

Spell level can be increased by using higher level spell slot, and I see no requirements that spell to be permanent must be cast by the same caster every time. Storytelling-wise, it would then make sense to invite archmage with 9th level slots to perform first or last casting for stronger, harder to dispel effect, and use “apprentices” for day-to-day casting duty. But is it mechanically possible? What dispel DC would such permanent circle or sanctum have, if spell slot level used would vary?