Adding a sump wouldn't be terribly painful. You'd need an overflow box, a return pump, and well...a sump
. For the overflow, CPR seems to be the most prevalent one in catalogs, but I have been warned away from them by several people. I have never used one. I have used a Marineland Tidepool overflow and did not like it at all. I think Big Al's has a basic U tube overflow for a reasonable price. The pump can be anything from a MagDrive to an Iwaki external. Depends on how much you want to spend and what you use for a sump. You can use a smaller aquarium. You can even use a Rubbermaid tub. Or you can buy an acrylic sump. If you use the aquarium, you will be limited to submersible pumps unless you have the tank drilled for a bulkhead fitting. The Rubbermaid could be drilled I suppose, but I don't know how well it would hold up. The acrylic sump is the best way to go, but will be expensive.
I like the astrea snails better because they don't move things around in the tank. I have a couple of the big turbos in my coral tank and they arrange the tank to suit them...always having to put rocks and things back. As long as you're not having problems with algae and detritus, I wouldn't worry about it a lot. You have room for more critters if you want to add them though.
If you're getting all those bubbles from the sand bed, then it is probably doing the job. Nitrates are staying low?2" is a little below the norm for a sand bed with nitrate control in mind, but it's not chiseled in stone anywhere that it won't work. Keep us posted on how things are going.