I do a similar thing, except that the email is actually hosted at my domain rather than being forwarded, and that I have a list of email addresses that I accept

I do a similar thing, except that the email is actually hosted at my domain rather than being forwarded, and that I have a list of email addresses that I accept and reject all others; if I receive too much spam at one address, I disable receiving at that address. I have found this to work; I hardly receive any spam at all, and do not need any separate spam filter.