AdGuard DNS and NextDNS both do the same thing, there’s some differences but they’re pretty much the same (except AdGuard public DNS that doesn’t need configuration). Rethink is the same but doesn’t require an account. Rethink and TrackerControl are also Android apps that give you control over traffic locally. It depends on what you’re trying to do but any of the DNS options (AdGuard, NextDNS, Rethink) will protect multiple devices.
I would use Telegram, even if there was no E2EE because the WhatsApp app collects more data. The Telegram app privacy section in the App Store says that it only collects data for “App Functionality”, while WhatsApp collects more data and uses it for ads, tracking, and “other purposes”. That said, both of these apps are bad and I would not send anything sensitive using them. With how bad Facebook is, I would not be surprised if WhatsApp had a backdoor. Even iMessage or RCS would be better than these apps (but of course Signal is the best).
Matrix is less secure than Signal. While Signal and Matrix use the same encryption, Matrix doesn’t encrypt everything. This includes: message sender, message timestamps, reactions, members, read receipts, etc. All of this data can be accessed by the homeserver admin. On Matrix, you should assume that only the message content itself (text and attachments) is encrypted. Your account data is also not protected, you have to trust your homeserver admin. Signal is designed not to trust the server. It’s important to consider your threat model. Matrix doesn’t require a phone number, which makes it better for anonymity, but Signal has better security.
This is a good explanation of Matrix’s metadata leaks: https://web.archive.org/web/20210618055112/http://serpentsec.1337.cx/matrix