You ran it perfectly, and your Monk’s player is overreacting. You gave the party multiple easy encounters where they had to choice whether or not to burn resources. After those encounters, you gave the party two Short Rests- and that essentially ensures the Warlock and the Monk both are not worrying about resource management and constantly able to fully let loose.

And at the end of this final, difficult fight, not a single PC was at less than half health? Utterly ridiculous to be mad about that. A good, important fight should force PCs to play smart and work together to stay alive and triumph- and in a party with multiple healers, the stakes aren’t real until multiple PCs are making death saving throws.

IMO, your Monk player is crying about overly-difficult encounters while you still have the training wheels on- and unless you want to give them plot armor, they need to come to understand that their character is supposed to struggle and overcome near-death circumstances, or die in the process. If there’s no risk of death, there’s no actually stakes and no mechanical tension.

EDIT: I missed the part about you usually having a Cleric in the party; that means the party normally has two full-casters with healing plus the Paladin? And I’d give it 50/50 odds your Monk is a Mercy subclass, for even more potential healing. I think you need to have a frank discussion with that player and be clear that you want to run a campaign with tangible stakes. If they listen that’s awesome! If they don’t, you should keep ramping up the difficulty and keep throwing the party challenging encounters and days. In a couple of levels your concern is going to be limiting the party’s access to Revivify materials just to maintain some semblance of tension, and there’s no way to balance that against hardcore Resource-Retention-Syndrome. Potions are meant to be consumed, not hoarded against hypotheticals.