>so if I can successfully filter spammers out, I think Google can as well.
This is absurd. The problems Google face are orders of magnitude more complex than your individual mail server. Your server benefits from the fact that no one cares about it. No one will develop a custom spam technique to send you spam. They will for Google. There will be whole offices in India dedicated to working out exactly what slips through Gmail.
You benefit from a kind of herd immunity where no one even sends the kind of spam that would reach you because it's blocked by Gmail so not worth sending.
The spammers don't attack mail servers, but individual mailboxes.
Your personal gmail account has the same risk of receiving spam as mine, but Google has much more tools available to detect spam. For example they can easily see somebody is spamming, when large gmail accounts are receiving the same message. They can easily see which hosts are compromised proxies as they receive tons of mail, and seeing large number of e-mails sent from questionable hosts should be easy to spot.
They didn't really improve spam detection much for for a decade, because blocking small hobbyist mail servers is more likely increase their user base.
Those special farms you're mentioning, wouldn't work, if filtering would be individual per user, as it should be, because each person's mailbox is different and each person has different definition what spam is.
This is absurd. The problems Google face are orders of magnitude more complex than your individual mail server. Your server benefits from the fact that no one cares about it. No one will develop a custom spam technique to send you spam. They will for Google. There will be whole offices in India dedicated to working out exactly what slips through Gmail.
You benefit from a kind of herd immunity where no one even sends the kind of spam that would reach you because it's blocked by Gmail so not worth sending.