ANSWER:

It’s mandatory upon you to first keep the missed fard fasts.

You can combine intention of fard missed fasts with Monday and Thursday fasts, Ashura, Arafah, 3 white days, Dhulhijja etc but not 6 days of shawwal. 

Sheikh, I try to keep Shawwal,Dhulhijjah fast.Bt I’ve around 65 fard fast pending because of not having knowledge of keeping them after getting puberty,I’m now keeping those by taking breaks often. Can I keep those nafil fast every year as I’ve fard left or should I first finish my fard fast as I’ll need long time to finish them