Yes, buying an used Pixel 8 or a 8a is what I would suggest. Its sustainable and doesn’t fund Google.

But their phone has some unique, inreresting features, like that Killswitches, 512 GB, expandable up to 2 TB with micro SDXC card, 12 GB LPDDR RAM, Electro-scrap positive thanks to the cooperation with our partner Closing the Loop, Designed in Germany/Falkenberg and fairly manufactured in our own factory in China , modular, easy replacement of Battery that has a standard format also used in other products from them,

A alternative to fairphone would be https://www.shift.eco/en/ They want to put killswitches into their new Phone which you can preorder for 535€ https://shop.shiftphones.com/shiftphone-8.html

It won’t be as secure as a Pixel with GrapheneOS. Grapheneos won’t support that Phone officially.

WhatsApp gives you the option to back up all messages to Google or Apple Cloud unencrypted.

You remember incorrect. All Clients are open source:

Telegram apps are open source and support reproducible builds. Anyone can independently verify that Telegram apps you download from App Store or Google Play were built using the exact same code that we publish

In Fdroid there are also forks. But yes, their servers are closed source and centralized.

Still its not recommended. It requires Phone number and as you said its E2ee is not on by default and is not soooo good.

It’s possible to skip on gmail if you create the account with a pixel without a simcard.