Just a quck check - can you confirm that:
A) you're filtering 24/7
B) that you're backwashing after psi increases 25% or more
C) that you presently have the skimmer valve open wide but the drain valve part way shut to allow upper layer clearing and debris collection?
Re:
The exact percentages, which I posted earlier as 7.5% of cya for regular sanitation and 40% of cya for slam derive from scientific research conducted by founding members of the forum. This information, which was an extension of research from 40 years ago, is now slowly filtering into mfg literature, but not consistently. To adopt TFC methods more or less requires an acceptance of the tenets of the FC/CYA ratio

But you don't have to take our word for it...just comb through the forum for explanations from ChemGeek in particular.
Mustard slam level is 55% of CYA. We recommend staying below that number to avoid fading liners. However, as I intimated in my first post I think, for very old swamp water, dosing between mustard level and slam level may expedite.
If you are not seeing color change of water, I feel there is something not going quite right and would like to see some fresh pictures and maybe have you hold a white piece of cloth in front of a return to determine if the water coming out of return is clear or has sand in it.
We've recently helped someone with a no-progress swamp resolve by determining they had broken laterals in their sand filter. So this may be a good check. It would help us to know if visibility has cleared at all as well.