Just catching back up on your adventure. Hopefully the deep cleaning of the sand & the lower/slower filtering will get things moving along.
About your cc’s - try cleaning your beaker with alcohol or ammonia free windex (rinse it well afterwards) sometimes old ucky stuff likes to stick around.
I see you also have deck jets & bubblers - are you also running slam level water through those for an hour or so daily? Any auto fill or overflow drain that may be gunky?
After that only stone left unturned is drain covers if you have them. The lack of overnight chlorine loss is a good sign. Try not to get discouraged, keep at it, replenishing slam level as often as you can