I have a LG G8 running Android 10. Your basic, unrooted, stock ROM phone commercial phone. It has a 64GB SD card where I place most of my data.
I want to know how I could achieve the following 2 goals simultaneously:
I'd like the files on the SD card to be encrypted so that if I lose my phone, the person who finds it cannot access the data on the SD card
If the phone dies on me, I'd like to be able to access the data on the SD card, simply by inserting the card in a new Android phone. Since it's encrypted, I'm fine with entering a password or decryption key that I would extract from my current phone.
- My future phone will be by a different manufacturer since LG has abandoned Android manufacturing. So no suggestions specific to my current phone.
- I don't want to set up proper backup for my SD card (eg using SyncThing or similar tools). I already know how to do backups, and it's what I'll do if there is no other choice, it's just common sense.
- I don't want to use 3rd party commercial apps or (more tolerable) open-source/F-Droid apps, though you are free to mention them for other people who find this question. I want a solution supported by the base Android OS (eg Developer Mode), or with standard tools like adb.
- I don't want to root my phone or change my ROM from the stock
There are other questions similar to mine (5 to 10 years old), where the answer is "you can't decrypt your encrypted SD card from your old phone". But in all those questions the OP has broken his/her old phone, or forgotten the password. That is not my case, my phone still works, and I can take action to prepare for the future.