As you say, pro-rating first charge has the content unlock problem and "anniversary date" of charge defeats aggregation.

TLDR: Aggregate on a single day, but pro-rate the second month, not the first, so there isn't a "runt" payment up-front.

I start a monthly $20/mo pledge on Feb 15th. I'm billed $20 on the Feb 15th, same day. This covers one month, Feb 15th - March 15th. Let's say the first of the month is aggregation day. Come March 1st I'm already paid up through the 15th, so I get billed for the pro-rated $10 for the second half of March. Come April 1st and all aggregation days thereafter I can now be billed on aggregation day for the full monthly charge.