It's not likely to be something deliberately caused by an update. Firstly, that would be commercial suicide. for the reasons you've given yourself, and secondly because you are still on Android 6: you've not had any system updates for a long time

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But random uninstalling of apps that are in internal storage is not an encouraging sign - internal storage is more reliable and longer-lived than microSD cards, but if you are seeing things like this I can't help wondering whether you are starting to have failures?
I would not bet on a manufacturer-specific backup being able to be restored to another manufacturer's device. But most manufacturers these days include setup/transfer apps to copy data from your old phone (usually via WiFi), and those are designed to copy from different manufacturers.
As for a phone where the microSD isn't accessible without taking the case off, surely that's a matter of the case design? Most cases do cover the microSD slots of phones. Unless you mean the phone casing, in which case don't worry, all phones these days are sealed units where you remove the microSD (if it has one) the same way as you remove the SIM, using a little tool (or paperclip) to pop a tray out. You will struggle to find a phone where you
can take the back off to change the battery, never mind one where you have to do that to change the microSD card
