Do you know any minimal Invidious UI you can run from a Podman/Docker container? Like just the frontend, so that I can use Firefox instead of that. I think having that as a base and adding the freetube features would be great.
Use your favorite search engine and you'll probably find something. Or self host a Piped instance. It's very similar to Invidious but imo even better.
Depends on what is secure enough to you. For me that is secure enough but I know a ton of people out there who would say it's not secure enough for them. So in the end it's up to you. Think about the risks and make a decision.
Yes. Your DNS queries will be sent to a group of DNS servers instead of just one and they all can respond. This helps lowering the latency and improves the reliability since not just one DNS server can respond to your DNS queries. The installation page of NextDNS uses the anycast IP addressess.
I doubt you'll notice a difference with a different DNS provider. There are 5 of NextDNS's DNS servers in Australia. As long as you use anycast you should always have good speeds and reliability.
Sadly Plex collects some data about its users. I remember opting out of some telemetry stuff but I can't remember where that was. If you want a self-hosted streaming service like Plex that completely respects your privacy, Jellyfin is what you're looking for. I tried it and it's okay but not as good as Plex imo. But if your main focus is privacy then you should definitely check it out. It's FOSS.
Edit:
I found where I had to opt out some data collection for Plex. Open this site, scroll halfway down the page. You'lle see two checkboxes for "Send playback data to Plex" and "Advertising Consent".