Thank you all for your help! I will try to reply to everything...
@brandon429 here is a pic of the tank a couple months ago:
I would gladly ignore nitrite and nitrate values if the tank did well but the issue is that corals are not fine at all, since the picture I've lost almost half the corals you see here, and the remaining survivors are not doing fine.
@BostonReefer300 about the food, something I forgot to add is I also tried different brands of pellets/flakes and it did not make any change (Hikari, Haquoss, Ocean Nutrition and others)
@Jekyl I agree, that's exactly my problem, I can't find the source at all!
@Azedenkae the 50 to 200+ reference was misleading, that was the entire range I observed during these 2 years but in the last months it has been stable at 200+ (over the measurable range of all test kits, even with a 1:2 dilution)
I've also tested the RODI water I use for changes (I use a 5 stages unit) and it was at zero both in nitrates and nitrites.
I don't remember the weight of live rocks I have in the display but you can see the volume in the picture above. In my sump I have Siporax and Pond Matrix, about 1l of each, and about 2.5l of Siporax, Pond Matrix and nameless spheres from Amazon in the Donovan's nitrate destroyer.
@Dan_P I agree about the possibility that some of the things I tried may have been too rushed to conclude their ineffectiveness but most of the things I did, I did for like months, especially carbon dosing. Is there something in particular that you think could have worked if given more time? Also, unfortunately, I am not making this up

and I also think we are missing important information, I just can't think what it could be.
@Nano sapiens I forgot to add that I don't have sand, I had a DSB when I first set up the tank but it din't work so I removed all the sand and added more dry rock after about 6 months from the first set up. Could the rocks have had organics stored inside that they are still releasing into the tank? At some point I was thinking of some heavy metal release but the ICP showed nothing above the range.