How to back up encrypted /data atomically in Android 12?

I accidentally upgraded my phone to Android 12 instead of a newer Android 11 and, to my dismay, it seems encryption was changed in a way that makes TWRP unable to decrypt my /data. For backups, I ran [borgbackup](https://www.borgbackup.org/) on /data from TWRP and would like to continue doing so. This strategy relied on TWRP being able to decrypt /data however. Is there another way to get: * Decrypted /data * No active processes modifying /data (ideally have it readonly) * Shell access * A writable rootfs where I can create a subdirectory to mount a tmpfs on I tried using Magisk to mount -o remount,ro /data in a running system but that didn't work: mount: '/dev/block/bootdevice/by-name/userdata'->'/data': Device or resource busy