Hey OP, your doing something right. Nice tank!Full tank shot.
The reason you don't want low NO3 and po4 is because 1) coral need a food source and 2) lots of times it opens the door to dinoflagellates.
You probably have some N and P in your tank because you are feeding the fish. Just because it shows zero doesn't mean it's actually zero. What we are testing for is the residuals left over in the water column.
If you do decide to up the nutrients, work on po4 first then no3. Lots of times doing it the other way around causes problems.
Do test your calcium as soon as you are able. I'm curious if it gets repeated low.
As said, many ways to run a reef.
Having said that, you don't have to run a tank by dosing. Many people do many people don't. It's up to you. You could probably do water changes for a while until you do get into some more SPS and they do start to grow. Right now just to what you feel comfortable doing. Your tank looks like you are doing something right.
Happy reefing