I am requesting a simple change here.
Instead of having to wait for the rotation or immediately doing the new Faction Delve in the Underworld. Of course, if you wait for the rotation, you lose all the Potions and benefits - Tiers.
I am requesting that you change how the Potion system works.
To simplify, when you buy a certain Tier, make that Tier available again if it comes, but make the option owned/unlocked for the Potions only, so that this choice is permanently active, just like the weapons are. You cannot craft more than one weapon in the Soulforge if you already own it, the same with potions.
You could make a new interface for the Potions, but I would keep it simple and not change anything.
To summarize it again, if you buy each Shop Tier purchase with Gems, you should get the Potion effect/s permanently active/unlocked any time after the Event for the Delves, a one-time investment.