I don't think your filter media is helping with nitrate removal, if anything it may be making it worse by trapping stuff and supporting nitrates.
Your nitrate removal is going to be through a deep sand bed, good porous live rock and the macroalgaes in your fuge.
40lbs of sand in the display is probably not going to do much since a fully functioning deep sand bed needs to be in the 3-6" range an err on the deep side if anything. My display is a 100G, 60"x18" and 23" high so is not significantly more surface area than your 55 is but I have 330lbs of Southdown sand in it for a 5" DSB. The more surface area you have at the proper depth the better off you will be. The sand in the 30G fuge helps but it does not have a huge amount of surface area. Personally I would transfer more or all of it to the larger display and maintain macro algaes and maybe live rock rubble in the fuge.
The grain size of the sand is also important, the finer the better when it comes to supporting both bacteria and the beneficial fauna needed to maintain the DSBs integrity. You want to disturb it as little as possible, never more than 1/4 to 1/2" deep when vacuuming or cleaning and then only in small sections at a time. No sand sifting starfish! They are predators and will kill and consume every living creature in the DSB making it look like the Sahara Desert in no time at all. Thats the biggest mistake I have ever made in my 20 or so years of reef keeping, adding a sand sifting star against everyones advice, I thought I knew more than them. Not! In 3 months I had 330 lbs of dead sand and it took a year of supplementing cups of donated live sand from friends, numerous bottles of live ocean pods and many many bags of live reef stew to get it back anywhere close to where it was. To this day several years later I still don't have the spaghetti worms or pods I once had.
Good porous live rock is important. In the beginning some of my rock was dense and very heavy meaning it was not porous and would not support bacteria growth way down deep inside the rock. Bacteria grows all thrugh the rock not just on the surface.
I have no filtration on any of my systems other than protein skimmers and media reactors for carbon and GFO. No sponges, floss or filter socks of any kind and never have. They all trap detritus and produce nitrates not lower them.
20-40 is not tremendously high for a FOWLR system if you have a heavy bio load like lots of fish. I have seen fish only systems exceed a couple hundred ppm nitrates and still do well, they wouldn't support any stony corals but fish did fine.
What are you using for water, hopefully RO or RO/DI water? How often do you do water changes and do you disturb the sand when you change water? try not to disturb the sand if possible and maybe increase the frequency or amount when you do water changes.