Gmail – Is There an ‘is:’ Filter for Sent Folder?

I would let one account handle the spam filtering for all of your accounts. I don't believe there are many benefits to having each account handle its own spam. I think Gmail is aware of the destination address of each incoming mail and having identical spam emails (with only the destination address changing) will strengthen the spam algorithm just as much as if it were single emails sent to multiple accounts.

Regardless of how much (more) it might improve the spam algorithm, you should choose the solution which best improves your workflow. I would just go with the first option (spam filter on the main account).

FYI: I use Gmail to process email from 12 accounts across 7 domains each with spam being primarily handled by my main Gmail account. :)

GMail is going to stop you from sending that many messages from their servers. They have spam prevention mechanisms

Likely, those messages did not come from Google's severs, as they won't let tens of thousands of messages go out in a day from your account through their servers.

Did you look at the headers people received?