Early Reception

I wouldn't fret over 30 minutes, especially if you're already planning on an after party. Biggest question is when you're doing your photography - 5:10-6:10pm will be your "golden hour" AKA the best lighting for pictures, so if you're not doing a first look or otherwise taking most of your pictures beforehand, that would be the best time! In that case, I'd bump everything back half an hour so you have basically your full cocktail hour for pictures.

Hi! We’re having an outdoor ceremony as well in December. Since we’re not sure if it’ll be a warm or cold winter, we decided to go with 4pm so our ceremony pictures will not turn out too dark. So during cocktail hour, we will take family/friends pictures with the sun setting in the background.
I’m hoping by 9pm everyone will be tired or would want to head to an after party close by. So I think it’ll all work out.

Hi ! I think it will be OK ! 30 minutes it's really not a big deal...

I agree! I don't think 30 minutes is worth readjusting the schedule for!!

As long as you've got sunset where you want it on your timeline, you're set!