I don’t think you’d need more than regular business service:
- postal worker comes every day, with pre-sorted bins and leaves with outgoing bins; businesses already do this.
- first you re-sort the pre-sorted personal mail to remove spam, preserving the post office sort to keep it by customer
- then you sort all the removed junk by sender you’re returning to
— and when your next mail arrives, you give the USPS worker the pre-sorted outbound mail of returned junk
- keep a Instagram of your piles of returned junk mail, every day
This already happens with, eg, university or large office mailrooms. So you’re not asking anything new if USPS to make it work.
Once a month, a service pickes it up, then leaves a dumpster of spam next to the local USPS mail drop. (Or shovels it over the counter at the post office.)
Also, provide a list of houses subscribed to the service to the USPS. The ball's in their court now!