Dear Daniel Friedman,
As next steps, I did replace sendripd=yes with =no for my trunks without causing issues which I could detect. With Core 3.0.14 and Framework 3.0.25, I could also turn on Firewall 3.0.11.1 without NAT/audio issues again. This probably does need practical testing for a few more days, but it does look/sound good, so far. I have Network Manager off and the interface configured through System Admin -> Network Settings. My interface is in the external zone and everything else is pretty much standard.
Thank you very much for guiding me to get to this point!
The only thing I could not get back after upgrading to FreePBX 13 is NIC bonding (NIC Bonding in FreePBX 13). It seems that it is now a must to configure interfaces through System Admin -> Network Settings and that does plainly not permit bonding. Is that correct?
Regards,
Michael