Don't give up. On the Fluval C4, is the top rattling? Remove the top and see what it sounds like (I have glass fish weights on my Aquaclear tops to stop the rattle).
I wouldn't downsize, but that's just me. You might find some value in
The Very Best Aquarium Filter.
In any case, I'm with
@Colin_T, don't give it away or sell the 55g cheap as you may regret it later. And smaller tanks may seem like less work, but smaller tanks really don't change the effort much and less water can mean bigger problems quicker.