Comparing a .22 to a .25 is like comparing a pigeon to a chicken. Two different beings.
Yes, using a heavier pellet "would be like using a .25" and I'd say try it. Could be what you're missing without requiring another rifle.
All the same, I believe you should try a .25, any brand, just to get a general idea of what one is like and then delve dealer into the brand you favour.
A .20 calibre I'm yet to see and try to have an experienced opinion but if you like features from both .177 and .22 the argument could still be heavier/lighters pellets than a new rifle.
However, lets say y