I think half feats aren't a substabtial reduced cost. It either relies on a careful min maxing at level 1 to have an odd stat, then requires that stat to be top tier for your build, or takes up to 8 levels of planning to pay off.
Second, I do not think good feats should be focused on one mechanical effect. They can be inspjred by a mechanical problem, but should remain story first.
Here the problem is "character that chooses to pick all fire spells".
Faced with a foe immune to fire, this character sucks, is near useless. And a fire mage sucking when fighting a fire creature seems a bit off.
The core of my feat is allowing not energy substitution - no "radiant ball" that makes damage types barely irrelevant - but admixture.
For this one (I have a bunch) we take the idea of hell corrupted fire. Ignoring resistance (because hellfire cares not for it), and for immunity providing half penetration by necrotic (because you cannot burn a fire elemental, we don't ignore immunity).
But feats should not just do one thing.
Temporary HP from the hellfire is a kind of life stealing. Then a damage boost if you aren't attacked.
While it is up to a 25% damage boost, it is intentionally awkward; it works on one target, and if you fired 5 rays of fire you get 25% of one of them on your next turn. You can't really get 25% basically.
Meanwhile, your fire spells are laced with corruption.
For other damage types the end goal - not useless against immune creatures - is paramount. A thunder/lightning feat, a poison/acid, and a cold one cover most of the most commonly resisted/immune types of damage.
Radiant and Force and Necrotic don't really need it.
Thunder/Lightning lets you split damage in half between types, and has a bonus kicker (thunder pushes, lighting kills reactions).
Poison/Acid similarly, split damage, kicker (adv on attacks/saves on damaged foes, and alchemist tools)
Cold you can split damage to do movement reduction, even on immune foes. So not quite the same; still useful. To play the damage game, you get a fire/cold reaction "protection" ability.