I am also a beginner and am setting up my second planted tank.
One thing I did right was to use soil under sand and I had really good growth, until my home made lights started to fail.
If you are having poor growth, you could use shallow pots filled with soil and placed under the substrate. That way you wouldnt have to redo the whole tank.
Point of reference, in soil my amazon swords grew from about 6" to 16" in about 2.5 months. And this was with cheap DIY led light strips. Also, my ludwigia grew more densely and deeper red.
I think the limiting factor in your tank is substrate. It is much cheaper to get some soil than it is to keep investing in root tabs. Your tank is lightly planted so pots under the substrate would probably work great.