I want to use fire-based spells for thematic reasons and a lot of things have immunity to it.
Is there a feat or class feature I could use to bypass/reduce those? For example, is there something that ignores fire resistance and reduces fire immunity to just resistance?
I know the easy way around it is just to pick another damage type spell but I really want to roleplay a pyromancer/firebender-type character.
Your best option is probably the Elemental Adept feat, as it is "permanent". But it only overcomes resistance, not immunity. This is probably the only really viable option in the rules as written, and it's not great, but it does best fit your character's needs.
So far, the only way to overcome immunity is the Pyromancer.
Note: Even though this is published by Wizards, it is unofficial content, meaning it is not vetted at the same level as official content.
The Pyromancer is a sorcerer from a Planeshift Pdf detailing the Magic:The Gathering setting of Kaladesh for use in 5e. At 18th level, they gain the following:
Fiery Soul
At 18th level, you gain immunity to fire damage. In addition, any spell or effect you create ignores resistance to fire damage and treats immunity to fire damage as resistance to fire damage.
There is the Elemental Bane spell. But that only covers resistance, requires a save and is concentration. Not a good option.
You can also look to spells abilities and items that grant vulnerability, as this is pretty close to the same as eliminating resistance (but again, it does nothing to immunity). Spells like Hallow and Contagion (flesh rot option) do this. Or Channel Divinity: Path to the Grave from a Grave Domain Cleric.
Any other ability I found is related to switching out the damage type, ala the Order of Scribes Wizard Subclass, which is not exactly what you want. Although, technically it does effectively bypass immunity to a given damage type. Check under the Awakened Spellbook feature.
Along those lines, in Tasha's Cauldron Of Everything there is a sorcerer metamagic option called Transmute Spell (remember any spellcaster can now get metamagic using feats) which reads
When you cast a spell that deals a type of damage from the following list, you can spend 1 sorcery point to change that damage type to one of the other listed types: acid, cold, fire, lightning, poison, thunder.
The Elemental Adept feat (PHB 166) says:
When you gain this feat, choose one of the following damage types: cold, fire, lightning, or thunder. Spells you cast ignore resistance to damage of the chosen type.
This allows you to bypass resistance. However, as far as I know, after looking through both the DMG and PHB, there is no spell or spellcasting ability that "beats" immunity, or changes immunity to resistance.
In the Plane Shift: Kaladesh pdf there is a pyromancer subclass for sorcerer. On page 9 in the bottom right. It has several abilities that are relevant.
Starting at level 6, all your spells ignore fire resistance.
Fire in the Veins At 6th level, you gain resistance to fire damage. In addition, spells you cast ignore resistance to fire damage.
At level 18, all abilities you use ignore fire resistance and treats immunity as resistance.
Fiery Soul At 18th level, you gain immunity to fire damage. In addition, any spell or effect you create ignores resistance to fire damage and treats immunity to fire damage as resistance to fire damage.
