You should ferment longer until gravity hits mid to lower 20’s. Also your hydrometer should say on it what temp it was calibrated at, usually 60 and it should include a correction chart so you can +/- Usually on a few points one way or the other
I did a kettle sour a while back with a Flanders recipe. Turned out a bit too sour for me as PH was target at 3.2 I believe but didn’t start boil soon enough to kill of the bacteria. Anyway I used a Wyeast bacteria to sour and yes a pH devise is a lot nicer to use.
