If 500 is working for you... stick with it. O'ing on Cd 18 or 19 is not late for a lot of people. PCOS or not. If you up your dosage you could give yourself a lot of unpleasant side effects and could even mess up your O. If you O regularly on CD 18 or 19, you DO NOT have a problem. If you start missing O, then you might want to look at bumping up your dosage.
Things sound good on your end. Just keep BD on the right days. After 6 months of doing it when you 'know' you are O'ing and no + you should go to your RE or OBGYN. It could be something else.
GOOD LUCK. You sound VERY healthy.
