For privacy, any ISP can technically see everything you do online including phone using wi-fi unless a device or syatem uses a VPN, Tor, or i2p, but that does not mean the ISP is keeping records of what an account does online. They may have no interest.

For security, you definitely need your own router. If you can build a system to use as a router running OpenBSD or pfSense, at leaat use a router with open source firmware so your router traffic is not going to someone else and an open source router will give you a lot more extensive network functionalities.

Does XMPP with OMEMO give you what you are looking for, or am I misunderstanding?

What would you say to someone who has no concerns about getting sick as all of humanity will forever get sick in the future, some will die, some will live, but life will always continue on regardless of a 100% guarantee of death for every new creation that is born? Every life will end, but life itself will never stop and will never end.

The fediverse has too many children who cry and throw a fit when someone expresses the wrong opinion, there are very few healthy function adults.

People who promote Mastadon and even Lemmy claiming freedom because of its federated services, is a lie they use to push tyrannical censorship for people having the wrong views, or wrong think, so they use services like ActivityPub where people are not allowed to prove them wrong, due to how traumatizing it is to read words on the internet that they don’t like.

No matter how bad centralized social media can be, at least there people have to be adults and handle criticisms and being corrected or challenged for what someone posts. There is no escaping all the evil cruel people who disagree and believe differently.

You can do that too, use an Android VM. Whichever way works best or works easiest for you.

I use Whatsapp in that exact scenario that you are describing. It’s an old Android phone connected to wi-fi and the phone is never touched for anything outside of Whatsapp.

Having it setup that way, or for you with FB/IG, if there is no SIM in the phone, do not install anything else, no contacts in phone, you can effectively use it anonymously.

I suggest you use an Android phone so you can download APK files to install, disable Play Store and similar network services, and maybe once a month download the lastest APK and install to update it. I do that for Whatsapp or updating since I have all Google services disabled on the phone.

Voyager is on both platforms, Jebora is only for Android, both are pure open source so you have full privacy and anonymity.