Is adjusting damage type an appropriate power level for bestow curse?

Question

A rather creative cleric in my game's party has presented with a new curse they want to use. The new curse is:

All damage the target creature takes is treated as the damage type to which the creature is most vulnerable

This curse basically removes any sort of damage reduction from the game, plus it can even make all damage the players do to monsters more powerful than intended. Our evoker, for instance, can memorize only powerful fire spells with confidence that those fire spells will have full effectiveness on creatures. The cleric is prepared to heighten spell to keep the saving throw as high as possible.

Is this curse:

1. Appropriate for bestow curse
2. Too strong for bestow curse but appropriate for bestow curse, greater
3. Inappropriate for a permanent debuff

I don't consider this a homebrew question since bestow curse explicitly encourages creation of new curses. I have just always had difficulty judging the power of curses

Answer 1

• −6 decrease to an ability score (minimum 1).
• −4 penalty on attack rolls, saves, ability checks, and skill checks.
• Each turn, the target has a 50% chance to act normally; otherwise, it takes no action.

These are the default, written effects of bestow curse. And any of them are “immediately stop whatever you were doing and find someone who can cast remove curse before proceeding to attempt anything else,” at least assuming the penalized ability score is one you rely on. Those are large penalties, and the third option is even worse than those are.

And in that light, this is... not that, honestly. A lot of targets won’t actually be affected—the default, after all, is that a creature doesn’t have any protections against or vulnerabilities to particular types of damage. All damage is bad damage and this doesn’t change that. For some targets, this is brutal, but even then, not necessarily something that needs to be dealt with immediately. There are plenty of things you can do without risking damage—e.g. anything outside of combat—and you could do those fine. That isn’t the case with the default curses.

On the other hand, this effect could potentially allow you to get past some ridiculous defenses. And in situations where it’s meaningful, and damage is a real risk, it probably represents a more-immediate threat to your life than the default effects. But when you consider all the other things a 3rd-level spell can do on a failed Will save, even that doesn’t seem like a problem to me. For a regular target I was just trying to kill, I probably wouldn’t bother, and for a target that needs to be debilitated in a serious way, that 50% chance of failure on everything you do is far more likely to be my choice.

But against that target with one crucial weakness, that we were having trouble targeting, I could see wanting it. And as a DM, wouldn’t mind it at all. Again, bestow curse can usually do worse.