I quite like Fastmail. It’s a bit expensive but the service is very reliable and they have a well established reputation. You can create masked emails using their domain or your own from the web interface.

As of the latest release (21), you can simply install microG on regular LOS and no longer need to install LineageOS for microG since it now includes the necessary signature spoofing support.

Invidious?

I have used all three! I started with Server then went to CoreOS running Kubernetes and settled on NixOS which I have been very happy with for about a year now. I run about 25-30 services all using built in modules.

Regarding security, if you are using well crafted modules on NixOS, there should be good systemd hardening in place. That being said there is no reason you can’t just use containers on NixOS.

I also find deploying NixOS far superior to butane/ignition used by CoreOS/Fedora. I use nixos-anywhere and can deploy my entire server in a few minutes without manual intervention.