Yet another way how to backup persist (or any other) partition even without PC, with rooted phone and Magisk: Install Magisk module "backup" and reboot.
After that using some terminal app in phone (I use Termux), first gain root access:
su
and then enter
backup persist
it will create file persist.img in folder /backup of your phone's internal storage. The path can be changed of course. I suppose same can be achieved with dd command, but the above is easier.
Instead of terminal app in the phone you can of course use adb shell from a pc.