Advice needed please

[Slick Fork]

Definitely get rid of the bioballs.

I would add some live rock to the main area of your sump, personally I wouldn't frown on putting it where the bioballs are but that's just me. I don't run filter socks, not because I don't think they work... I'm just not willing to change them every few days and if you can't change them then they become problematic.

The gate valve is an excellent suggestion on a herbie, way more "tunability" of the water level in the over-flow. I have a herbie setup as well and couldn't imagine how a ball valve would work properly.

Heater, skimmer, and any other devices you don't want in the display can all go into the sump.