is there any way to improve ability score withitout items, olny whith spell or class features?

I found a few, But all is bad, like true polimorph to improve ability( but i lose class from game statistics,), someone know any way, to improve without lose class from game statistics, and without use books and tomes only spells and class shenanigans?

I think wish spell it’s a bad way way, but it’s a way.