I self host with Synology Photos, pay for extra Google Photos storage, and just today started using Proton Drive, but Proton will just be for whatever is on my phone today and moving forward. The other two have my entire library.
Explain setting up a Raspberry Pi to your aunt and have her tell me it’s a low pricw. Let alone lineage os, Jellyfin, Smart tube, and whatever else was in your list. And that solves just casting and steaming. Now do email, messages, browser tracking and fingerprinting, and everything else in our lives… It a lot. And honestly too much.
I’ve never looked into backing it up. I do know they make it easy to import emails from other services so maybe they’d make it easy to export to other providers too? You’d have to look into it.
I use Authy and am logged in on multiple devices so if I lose my phone I can still access the 2FA on my laptop. Then log back into the new phone using the laptop.