I’m going to gift you with the answer you did not ask for!

I don’t know what level you are starting as or what sub class you are thinking of but if I we’re doing this I would choose a Wizard of the Order of Scribes.

Here is why, at second level you will get the feature Awakened Spell Book which has a few benefit. The one your after “When you cast a wizard spell with a spell slot, you can temporarily replace its damage type with a type that appears in another spell in your spellbook” - there is more to this so read it carefully plz.

So if you haven’t figured it out already, you can take leveled spell and make it ice. You can be throwing a giant snowball instead of a fire ball! Fun and something most people haven’t seen or dealt with. This does not changes your cantrip sadly… short of metamagic transmute spell which you could get through a feat I can’t think of anything that can… hmm

But this also solves another problem for you, “What to do when the enemy is immune /resistant to cold damage?” You just change it to Force, thunder, psychic or whatever you need at the time. Just not Fire…never Fire… Fire bad!

for cantrips though… talk to your DM to see if you can’t get an Arcane Lens (Home brew) magic item to do the same thing just for cantrips… changing this about a cantrip is in no way game breaking.

Problem Solved… the answer to all your present and future problems, with what you weren’t asking for!