I don’t know that it’s really necessary to use both nextdns and pihole. You may look at a couple of comparisons and decide what’s best for you. I just use pihole (two of them actually, one in docker and one on an actual pi).

It’s the same thing. There’s an option before you sign into the extension to choose a different server.

Yep it’s for all my linux ISOs. I have it in my homelab. I probably WAY over-complicated things but I use OPNsense for my firewall and selectively route traffic from specific containers down a ProtonVPN tunnel. I’m using macvlans within docker to give those containers dedicated IP addresses which allows the selective routing to working.

Yes it can, though it is easier to set some things up with the built-in addons. Most addons can be set up independently as docker containers (like z2mqtt or node-red) but may require additional configuration.

Not by default that I am aware.