Baby survival rate is the hardest part of this. A lot of hobbyists can get berried shrimps, but a lot less get good baby survival rate. So you're not alone.
I had that issue before, on counts of both CRS and OEBT. I have seen hundreds of babies released but very little survived.
Then I set up different tanks with different substrates (only significant change), and I get pretty good survival rate now. Not saying your choice of substrate is bad, just share my experience. I wasn't using FSS before anyway.