Hi @NLS
As you’re not unfamiliar with Linux, and are running KVM, my stopgap (to allow you to proceed your plans as planned) idea would be to spin up a dedicated getmail VM or container, which feeds the mail via port 25 into NS8.
This would allow your sieve filters to work as planned, and your front-end / back-end to work too.
Your mileage may vary (But you will get there!)
My 2 cents
Andy