Requesting thoughts on balance of homebrew ability

[Adamantly d20]

1/day + Int
May expend an additional spell slot to increase the Save DC of the spell cast by an amount equal to 2+ the level of the spell sacrificed. At 11th level you may use this ability to expend 2 slots gaining half the bonus from the second additional spell expended in addition to the original bonus.

Balance thoughts? And suggestions.

[Buufreak]

What is this for? A class? Prestige class? Feat? Domain power?

[Adamantly d20]

Either feat or dedicated class feature yes/both/either

It is supposed to be similar to how psionic abilities can be more powerful by expending more power points.

[Fizban]

So you basically want to let people just double their spell DCs? 'Cause that's what this is. Spell DC formula when using this is 12+ 2x spell level +ability score, which is. . . no.

Psionic augmentation is something spells already get for free from the effects of increased caster level (its what makes spells better overall, free power on lower level slots). Psionics that are augmented to maximum have the same DC as the highest level spells cast by a spellcaster of the same level, because you can't spend more power point than you have manifester levels (aside from a couple abilities that let you go a few points higher, which would be equivalent to +1-2 DC, not this). Casters who want their low level spells to have higher DCs can already take Heighten Spell to cast them in higher level slots with higher DCs.

[Goaty14]

Finally, something useful for cantrips...

I wouldn't do this in practical optimization unless you had to expend a same-level slot because then you wouldn't do it to your high-slots, but would for your lower slots, which is kinda like psionics.

Still pretty unbalanced though. A 8th level wizard doing 2 4th level slots has a DC of 10+4+6+INT, which is about a save DC of ~26.

[Rynjin]

Could be balanced a bit with some numbers tweaks. A minimum +2 (or 3, if you disallow cantrips) to DC is way too strong, and max +11 likewise.

Maybe if you had to expend the same level of spell as you're casting, and got a scaling DC boost (levels 1-3 +1, 4-6 +2, 7-8 +3, and 9ths can be used for any level and give a +4?).