I would never have thought that blocking just one country and one ASN would be so effective in reducing the spam reaching my email server, in addition to using Postscreen and Rspamd, of course. Over time, I had built myself a pretty sophisticated filter with Sieve and added a long list of domains to reject, but automatic blocking with iptables is even better.
Now I have this setup: automated blocking of country/ASN ranges with iptables + Postscreen + Rspamd with Abusix integration + Sieve rules. And my inbox is quiet again, and only legitimate mail reaches me.