I would call 20% every other week kinda minimal for a new, heavily stocked tank. I do 10% every week, though after my last tank was up for a year I started to feel like I could back off a little. I hadn't had nitrate or algae issues for several months so I was comfortable stretching it out to 10% every other week or so.
It's tough with a 55 because you are generally restricted to hob equipment, which tends to be less effective as it sacrifices on ability in favor of space and noise. And when you're already cutting it close, each extra fish is another poop factory. FWIW I don't agree with feeding less as a nutrient control measure. The downsides of weak immunity and vigor and coloration make it not worth it to me. You don't want to overfed of course, but you need to provide sufficient nutrition first, and deal with export second, IMO.
When you are addressing accumulated nitrates, it can be effective to change a lot of water in a relatively short period. Like a reset. So, your nitrates are 40 ppm, and you do a 20% change, if you take 20% out of 40 you still have 32ppm. Not so great. But if you do another 20% change the next day you are close to 25ppm. Much better. Now, that's not accounting for the nitrates that are being added every day by your poop factories, but you get the idea.
It's very important to match temperature, salinity and alkalinity when you do more than 10% change; and it gets more important the larger the change. I can use cold water for a 10% change without a measurable effect on the temp of my tank. I did that one time for a large (I think 30% or so) change and killed 2 fish.
Once you knock them down they may still not be lowered much by water changes, but if you are lucky they will be bouncing around below 20ppm instead of 40. That's better for your inverts and eventually the bacteria that eat them will hopefully get more effective.