I avoid dungeon.wiki and dandiwiki like the plague, everything I see there seems horribly unbalanced and poorly thought out.
Those feats are actually not that OP, albeit really dumb, IMHO. But your reasoning is unsound.
1) IME enemies make saves freaking often, it's the reason no save spells or ones that do something decent even on a success are so popular (well, the biggest reason...), there is no crisis of unbeatable saves. I'm strongly considering a tailored RAISING of save DCs to make certain types of spells more appealing compared to others. Direct damage getting a massive save boost, for example. Or save or die getting a penalty to make them less effective. The current system is too static, so you end up with spells like Unluck or Mind Fog that do cool things on a failed save, nothing on a success, but people respond "if I can get them to fail a save for that, why not just use a spell that will end the fight?" If the save DCs weren't the same for those spells, then that wouldn't necessarily be equatable any more. /rant
In any case, those feats don't help the problem, because the best way to screw someone over on a save spell is to target a weak save. Weak saves are almost always weak saves because it has a poor base progression for the creature. Your feat boosts the base component, and so will have neglible benefit for those who really need it. It's actually better as a fail-safe on a strong save in case of rolling a 1.
2) You could make the feats open to any class and thus avoid the gamist element you inserted, and I can guarantee you most full casters won't bother to take any of those feats and those that do would be nerfing themselves. They have better options already.