If you use gitea, it’s just a few steps to enable it to be an OAuth2 provider. See Oauth2 Provider Docs

Personally, I will use both: On servers with fixed network connections I will tend to use ifupdown; but on my linux laptops I’ll use networkmanager or networkd which tend to have nice UI’s for joining various forms of wifi networks. On my laptops for some VPN’s i"ll use the ifupdown configuration, which lets me setup all sorts of exotic configurations (bridges, vlans, vxlan, vpns, namespaces, etc.) The linux command line tooling has a litany of functions to check/test/diagnose/tweak networking settings, and they work across all the distros, AND they can reveal the full details of the network, as the kernel sees it. NetworkManager, networkd, connmann, etc, often omit details in the name of simplifying for the most common scenarios.

As an anecdote – I have been sitting on an elastic IP at AWS for years, with reverse DNS configured properly for it. Way early on (years ago), some spam filters would block the whole netblock, but I can’t remember the last time the IP Block was wholesale blocked. I think AWS is very much on top of any spam complaints from their Elastic IPs, and as long as you don’t abuse your specific IP, you are in good shape for light volume, non-spam mail.