Is there a way, barring Epic Boons, for a player character to gain full immunity to fire damage. I know it's easy to gain resistance by, for example, being a tiefling or casting a spell that grants resistance. But I'm looking for full immunity. The lower the level at which it can be accomplished the better. Ideally I do want to maintain all of my character stats, just with immunity to fire damage. Magic items are acceptable as well. I would like to have as comprehensive a list as is reasonably possible so I know what all of my options are.
Invest yourself in fire
The spell investiture of flame includes the effect:
You are immune to fire damage and have resistance to cold damage.
So simply cast this 6th level Druid, Sorcerer, Warlock, Wizard spell on yourself and you become immune to fire for 10 minutes (assuming your concentration holds). Things around you might get a little toasty though, but that's not your problem.
The 6th level Druid spell primordial ward, gives resistance to damage type and you can use your reaction to get immunity to fire damage when you take some (the spell then ends at the end of your next turn).
17th-level Forge domain Clerics get permanent immunity to fire damage as a part of their Saint of Forge and Fire feature.
The red dragon mask, a legendary magic item from Rise of Tiamat, can upgrade a fire resistance you already have to immunity.
Efreeti chain (a legendary chain mail) grants the wearer immunity to fire.
If you slay a fire elemental while attuned to a ring of fire elemental command it also grants you immunity to fire damage.
The unofficial Plane Shift: Kaladesh document includes a pyromancer Sorcerer subclass (p. 9) which at 18th level gains immunity to fire damage.