How to improve coral growth?

You need to get rid of the glass cover. Having to top off is just part of the hobby. You can always automate top offs... but everyone has to top off daily in some fashion. You ask how to make corals grow/split... this is part of it. The gas exchange isn't happening properly and it's going to contribute to hindering growth.

That space between the water and glass is irrelevant to everything. That 2" gap isn't great. It's going to impact your pH and that's going to impact your growth.

Kent doesn't make a single product that's worth my money IMO.

Coral grows slow. Just how it is.