I too would drop the cannister filter, though many use with out issue. It's a question of maintenance to keep them clean all the time.
Home made rock will never come close the porosity of Pacific coral rock, either base or LR
If the hydrometer is a plastic swing arm type, get a refractometer. The cheap plastic hydrometers are inconsistent and therefore inaccurate.
I'm not sure where the idea of putting a DSB
in a sump came from, but it's a bad idea. There is just not enough surface area to do any good as far as nitrate reduction. Now if like some your sump is the size of a kiddie pool in the basement, then I understand. Perhaps that's where the idea of a remote DSB
Good luck with your new adventure.