I'm a member of a Discord server like that. It's named Past Gen. This is the server's own introduction:
Welcome to Past Gen Gaming! We're a community of generally older, but mainly patient and relaxed gamers. There is no age requirement to join us, just the mindset. We won't tolerate elitism, toxicity, nor any form of hatred. A lot of us don't have a lot of free time, so when we do, we like it to be a good time. So sit back, relax, and let's play some games together.
Just let me know if you'd like an invitation link. If so, I would DM it to you instead of posting it publicly.
Mastodon is a huge part of the Fediverse. It allows you to control who can follow you (you can set it to prompt you to approve each new follower). You can make your posts visible to your followers only. Each post you make has a visibility option, and you can set it to followers-only by default.
Protective DNS, when set up with a DNS provider that blocks known ad / tracking domains, would help with that. NextDNS, Control-D, and Mullvad all offer this service, for example.
I think you'll be fine as long as CalyxOS is supplying your device with Android security updates. As an average user, with no reason to be the subject of targeted attacks, firmware vulnerabilities are not a huge concern (assuming your OS and other software are up to date with security patches).
Of course, if someone hostile gets physical access to your device, firmware becomes more important. Remote exploitation of a firmware vulnerability typically requires first exploiting a software vulnerability (and CalyxOS is updating your OS software). With physical access, one might skip that step by connecting a cable to your phone and interacting with it directly.
Firefox's built in "Enhanced Tracking Protection" already does a lot of this work for you, especially if you select "strict mode." Go ahead and try this extension if you prefer. But remember to be selective with the extensions you add to your browser in general, as they also present an opportunity to introduce vulnerabilities.
Done!