Existing potions versus new potions
Sozu supplies one additional energy at the start of each turn but prevents obtaining more potions. It does not describe removing the potions already carried. Those remaining consumables are a finite reserve; their replacement value changes after taking the relic.
Compare the extra energy with what the deck can play consistently. Energy can improve several turns, but it does not draw a missing answer or supply emergency protection by itself. A deck depending on potions to survive a bad opening pays a meaningful cost when that safety valve cannot be replenished.
Potion-generating effects also lose their intended refill benefit. Entropic Brew is not a workaround for a restriction on obtaining potions, and extra slots from Potion Belt do not themselves create usable potions.
The decision is therefore broader than whether the belt happens to be empty. Evaluate the current carried potions, the fights remaining and the deck’s unassisted damage and defense. For how to value an early potion against a later boss, read when to use potions, then adjust that comparison for a run that can no longer refill its supply.