Personal opinion having posted 17K times, and likely read double that number of threads on TFP:
I would not FLOC.
The first thing I would do is get an OCLT done. I'd like to know where you stand with that. We may have a solution, IF you can pass a couple OCLTs.
If you are not passing OCLT, then are you handy? Can you DIY PVC?
I would think through and plan to fix the filter. Have all the PVC, unions, elbows, etc. We can work through a plan from what we see.
I would deep clean the filter, and add sand to get it back to proper levels. If you are a newer owner of the pool, you might consider replacing the sand, you don't know what junk the last owner put in the pool that mucked up the sand.
Then I would replumb, properly, so that you are in a position, with proper unions to be able to work with your equipment.