3.5 mm is already big to me, it's pediatric sized, I wonder even how someone manage to get an adult foley in their urethra by themselves, even with lube. ellargement maybe...
What size/kind catheter are you using? I'm into sounding and urethral play. I'm maxed at 8mm. I can do more than 8mm but not 9mm. That's not worth splitting my dick like a lot.
Very carefully, lot of tries ending in the bladder, if it hurts or its too much discomfort, I don't go further. I see what comes out with a syringe, and reaching it is seen with clear fluid filling in. It's very technical.