My first suggestion is reduce the depth of sand. Get it down to 1-2 inches max. My experience with sand beds is they either need to be close to 1" or go DEEP with a full 4-6". Each sand setup has its unique maintenance requirement.
I can't tell what you have for filtration?
If you are moving more than roughly 50 gph across the Seachem Matrix, it's not going to do much for nitrates. Matrix is excellent for hosting both aerobic and anaerobic bacteria, but the anaerobic bacteria need to thrive deep inside the Matrix pieces in absence of well-oxygenated water. If you are pumping a high volume of oxygenated water across the Matrix media, then it's not going to be very effective processing Nitrate.
I would be very tempted to suck out ALL that old sand and start over with some brand new sand at 1" depth. Be sure to "feed" the tank with some source of Ammonia (a fish, fish food etc.) and keep dosing bacteria daily for a period of 7-10 days. You should be able to watch the nitrogen cycle move through from Ammonia to Nitrite to Nitrate, then watch the Nitrate levels to see if they ever drop, or just sit there and slowly climb.