Hello @pythonista2,
There is Postmaster Tools – Google which will tell you why the sending domain has a low reputation. You will be able to see (maybe after a few days when enough data has been collected by Google) how many messages are forwarded and among those who many messages Google considers to be spam.
Virtualmin has two methods of forwarding mail. The forward option in the Edit Users screen will forward mail without filtering it first via Spamassassin but the forward option in Usermin will first reject spam and then forward only non-spam messages.
Maybe the latter option will help repair the domain reputation that you have with Google.
Oh, like you, I too programmed in assembly language - for the 6502 way back in my teens in the 80’s.