Welcome. Your tank size is pretty close to mine. I change out 5 gallons a week. The easiest way is with a maxijet 900 pump lowered into just the top of the tank with a hose going to a 5 gal collection bucket. Don't throw this water out until you've mixed your new water and got it into the tank---in case you mismeasure and have to add some back. This means 2 buckets.
To mix your salt, add 1/2 cup per gallon of water. Use the same little pump to mix with: just drop it in the bucket with the water and salt and let it run a couple of hours to get it mixed and dissolved.
Test the salinity of your tank (refractometer recommended) and match it with your new water. Warm it to something close to your tank water. A second heater will help here.
This all gets a lot faster as it becomes routine. Keep towels handy while you're figuring it out.
You asked about feeding: it sounds like way too little, but honestly, if each fish gets one good mouthful a day, that's it. They'll beg until they're stuffed like turkeys, but it will do them harm: what goes into the fish goes into the tank, and if the tank can't handle it, it will throw the entire tank chemistry into a fatal tailspin. If you ever have to leave your tank and let someone feed your fish, parcel out the food for them. Everyone responds to the begging and overfeeds, and it's terribly bad for the fish. The urchins will eat what passes through the fish and ends up as algae (fish poo and lost food = phosphates, which act as algae growth booster). Your inverts are in there as cleaners and sanitation engineers. They don't need fishfood of their own.
Good luck to you.