Can I recover bricked device from Mask ROM mode?
Great to see you found a way out 
Pine64 has not provided factory image, as far as I know, but I would bet they're just using the standard Manjaro/Plasma image, because 1/ everything can be changed on a linux system, you don't get protected storage, so any change made by Pine64 would be at risk to be overwritten by a further update, unless they decide to publish their own distro, and 2/ that would not be required anyway, given both Manjaro and KDE are specifically targetting the Pinephone. So it's already made to work with the Pinephone/Pinephone Pro, and if Pine64 wants a change, they can just submit a patch upstream instead of having to fork anything. Just an educated guess, of course.
Note that even if Pine64 indeed just flashes a public Manjaro/Plasma images and you do the same, you will only have the exact same state as when you received the phone if you flash the same version of the image, which would require some guess game. I don't think that would be very useful either : you may want to do that to revert to the last known working state, but it was not working in your case.
I agree with Conjada that you may want instead to wipe out the eMMC and install the OS on the SD card (you can still set up a filesystem on the eMMC and use it as storage, the wipe out is just about deleting the boot partition), because it would make testing various OSes slightly easier : you don't have to hold the RE button each time you want to boot on the SD card. But no big deal if you install on the eMMC, you just have to remember to use that button, and you may not try new systems that often.
Pine64 has not provided factory image, as far as I know, but I would bet they're just using the standard Manjaro/Plasma image, because 1/ everything can be changed on a linux system, you don't get protected storage, so any change made by Pine64 would be at risk to be overwritten by a further update, unless they decide to publish their own distro, and 2/ that would not be required anyway, given both Manjaro and KDE are specifically targetting the Pinephone. So it's already made to work with the Pinephone/Pinephone Pro, and if Pine64 wants a change, they can just submit a patch upstream instead of having to fork anything. Just an educated guess, of course.
Note that even if Pine64 indeed just flashes a public Manjaro/Plasma images and you do the same, you will only have the exact same state as when you received the phone if you flash the same version of the image, which would require some guess game. I don't think that would be very useful either : you may want to do that to revert to the last known working state, but it was not working in your case.
I agree with Conjada that you may want instead to wipe out the eMMC and install the OS on the SD card (you can still set up a filesystem on the eMMC and use it as storage, the wipe out is just about deleting the boot partition), because it would make testing various OSes slightly easier : you don't have to hold the RE button each time you want to boot on the SD card. But no big deal if you install on the eMMC, you just have to remember to use that button, and you may not try new systems that often.