So do you think I should let FC drift down (if so how low?) so I can more accurately measure pH (like I should have done at the start)? Then once pH actually MEASURES properly, start a SLAM again? Otherwise I guess I'm just "guessing" at pH if my FC is too high.You actually might need to do this: Deep Cleaning a Sand Filter
With your higher TA, that should be okay, under usual circumstances, it's better to not aim lower than 7.2 with vinyl.
You have FC reading, but it matters what your CYAis... The (main) reason for seeing the bottom is safety (and health), as long as kids aren't swimming, you are likely ok for cleaning purposes.
When did you add CYA? Also how did you add it?