The need for water changes depends on a number of things.
Water changes export waste from the system. But they are not that efficient at it. To halve say excessive nitrates, you have to do a monster water change of 50% and even that will not hack it with levels that you have to halve and halve and halve again. Some other form of more targeted export will be more effective.
Water changes will also replenish depleted ions like calcium. But again, targeted dosing is more effective.
But water changes are a good brute force way of helping to maintain you system. They both export and replenish. A 10% water change three times per month is a good practice.
Nice tank! Looks like a lot of softies and some LPS corals that are all pretty happy. I'd say what you are doing now is about right. You could get away with a little less even, but why mess with something that is working. Watch the calcium, alkalinity, and magnesium though. If they drop between changes, you might increase the number of time you change water of the amount of water changed.