I have a fender mexican strat, with 10s. I have never changed string gauge, I only tuned all the strings down half a step to Eb from E.
The issue I am running into is that the low Eb, Ab, Db, and Gb strings are all sharp. I tested the intonation by playing the string open and then checking the 12th fret. These 4 strings seem to be in tune when played open but all of them are sharp when played at the 12th fret. The low Eb is the worst and the rest of them are just slightly sharp. I tested this using consistent attacks on both open and 12th fret.
I tried to fix the issue by adjusting the saddles. I made the saddle adjustments in increments by moving the saddle towards the bridge after loosening the string and it seemed to only make the Eb more sharp. I also got the point where I can't physically turn the screw further as it is very tight and I don't want to strip the screw.
I'm not sure what to do now. I have very little experience with guitar set ups so I wanted to see if I can get any advice on how to proceed without damaging something.