I would agree with lost66. I currently have a 150 g mixed reef system. I also have a sump and fuge. Everything is healthy, corals are growing well, and there is no issue with undesirable algae.People spend fortune to achieve 4 years old stable tank. If you restart your tank and will have the same setup and the same habits you will have the same results. Moreover you will struggle to achieve a stable environment you worked so hard to achieve.
I am not an expert but I read in few places that phosphates bind with rocks/sand, nitrates are diluted by water changes. That being said I think you just have a higher input than output of nitrates.
To check that theory you can do a simple experiments. Check nitrates before water change, do a 20% or so water change and check nitrate level again. Is it the same or lower?
Second experiment - remove 1 rock and put it to a bucket with fresh batch of water. Put the rock there, wait few hours and check the water. Is it super high or low?
I would focus on your sump, increase nutrition export by chateo/turf scrubber, roller matt, filter socks etc.
People chase numbers but do you really have to lower nitrates? If you introduce drastic changes you may do more bad than good. There are people with a lot of sps corals with high nitrates. I have some basic sps corals and I don't even measure nitrates anymore.
Nitrate (Hanna high range) runs in the 40's. Phosphate (Hanna) runs between 0.05 and 0.10. This has been stable for almost a year in a 3 year old system. 15%-20% water changes result in transient proportional decreases, yet nitrate never exceeds 50. Freshly made salt water with RO/DI water reads 0 with the Hanna low range. All other parameters are typical for mixed reefs and also stable.
So, I watch and do little. I recently decided to try Brightwell Aquatics Xport NO3 cubes (https://brightwellaquatics.com/products/xport_no3.php) in combination with their MicroBacter7, which, if it works, should slowly remove nitrate as it populates with bacteria. Fortunately, my fuge has an independent flow which is slower than the rest of the sump. According to the directions, the lower the flow the better. If it takes down nitrate slowly (over weeks to months) into the below 20 range, I will sleep slightly better.