Matrix / element

Jabber (XMPP) or Matrix/Element.

My friends and I all use Simplex and have been very happy. There's an annoying feature where messages in group chats are sometimes delivered out of order, but it's easy enough to live with.

"I remember my uncle, he used to force us to use some janky app to talk to him because he was afraid of the big corporations spying on him"

SimpleX.

Try SimpleX chat.

Edit: or session

https://simplex.chat/ is probably the best option today

Matrix would be good ig.

XMPP.

Threema.

Matrix.

You can use a voip/sms service to register with the traditional messaging apps: signal, telegram, etc

I.e. google voice, or https://kycnot.me/search?q=sms&type=

Though I think :

Matrix, simplex, and briar are good options that don't need sms at all

Signal v7 with username IDs is right around the corner. I'd suggest you wait a few weeks if you can.

FYI, last week someone saw that Signal merged in code for username (no phone number) support. So it might not be long until you see a beta release which allows you to sign up without a phone number.

Threema.

It costs $5 dollars, so you’re less likely to be the product. Highly private. Includes some open source code. Audited. Android and iOS. Doesn’t require phone number.

Here are the settings I recommend during first setup: Tap next instead of making a Threema Safe password Tap next instead of entering your phone number Turn off “Sync contacts”

I've used a self hosted Nextcloud for that. It's not great but it worked well enough that I didn't bother looking for better alternatives. I'm sure there are more options out there.

With Signal you only need a SIM card to register the account via the Signal app on the phone. You can receive the SMS for registering on another phone. After registering you can simply use Signal over WiFi. If this is too difficult, XMPP with a good OMEMO client for E2EE is an option. Easiest to install and start with is perhaps DeltaChat, E2EE chat via email : https://delta.chat/en/ You only need to learn what to do if the E2EE is not working right away. They have a FAQ and hints in the app.

You can also try Jami.

https://jami.net/

jisti, nextcloud talk, element matrix ...go big: IRC

I'm aware of the community this is posted in, but my kid talks to her grandparents via the chat features in Google photos. Works over WiFi with specific contacts and no phone number required. It lets her show them pictures she's drawn.
Google are involved of course, but there's no algorithm pushing promoted posts or any of that BS

Signal is no longer requiring phone numbers as of V7, so you could just wait for that to get released.

Session, but if you're really paranoid ofc.

If you want to use imessage on android:

https://www.wired.com/story/how-to-use-imessage-android/

(includes self-hosting options)

If you want to use whatsapp:

If someone else has an old (and/or prepay) sim which can receive texts, you can authenticate whatsapp that way. IRC you can also authenticate with a landline. Install whatsapp using the landline number, wait for SMS authentication to fail, then it should ask to call you. Say yes, and you'll get a call with the authentication code. Et voila.

My nieces and nephews are all young children so i definitely want to encourage/educate them to get off corporate controlled communications channels

Good luck with that, is all I can say. Kids are very susceptible to peer pressure, especially as they enter their teen years. It's only when they're in their late teens that they'll open up again and start becoming more critical.