You have a few not very good options for overcoming resistance, and only one option for overcoming immunity.
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. 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:
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 from a Grave Domain Cleric.
Any other ability I found is related to switching out the damage type, ala the Lore Wizard, which is not exactly what you want. Although, technically it does effectively bypass immunity to a given damage type.
Along those lines, in Unearthed Arcana : Class Variants there is a sorcerer metamagic option called Elemental Spell. Note: this is playtest rules.
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, thunder.