PSA - A crucial detail about Fastmail's spam filter that should really disclosed more clearly is that your filter is only trained on email you flag from within the web UI. Invoking your email client's "move to spam via IMAP" function does not train the filter.

I found "aggressive" was too much (which if you look at the custom option, sets the scoring to 4). I am running with scoring = 5 on the worst account and it misflags some emails - ironically it tends to misflag emails that are coming from domains where I get a lot of valid email from!

Re forwarding, there is a setting to address that in the same location:

Fastmail > Settings > Filters & Rules > Filters > Spam protection > Advanced settings

"If forwarding email to Fastmail from another service, enter the SMTP host names (not IP addresses) of the forwarding servers. The Received headers of these hosts are parsed to find the true email source. Please contact support if you require assistance with this feature."