[Android Q&A, Help & Troubleshooting] DavidxxxD:
Glad to hear you already got it unbricked!

For the sake of it being an intresting case, here is the more complicated way I put together and what I could find out from looking at the attatchment and a payload file for your device:

Good news - Your device is not too badly bricked.

Like some other new devices, this phone does not use a cache partition. That is why the recovery has no option to wipe cache.

The "invalid sparse header" message gets triggered when using fastboot to send a filesystem image that is not in the expected android sparse format. This can safely be ignored as the images for your device are uncompressed and the program doesn't know what they're suppsed to look like.

The NX669J uses a dynamic partition and the one on yours appears messed up in a way where I have no idea how it got to this point - all partitions for the A-slot are missing and instead there is a "system" (not system_a) and "system_b-cow" (!?) - This is probably why the bootloader is freaking out.

Usually, something like this can be fixed by just reflashing the super partition. However the update payload does not seem to contain one, so - unless you have a super.img from somewhere else - you would have to manually fix the current layout.

- Put the device is fastbootd mode

fastboot reboot-fastboot

Note: To verify, you can run

fastboot getvar is-userspace

Note: Didn't expect to have to say this, but don't run these commands on devices with correctly assigned partitions, unless you know what you are doing! They can be used as an example of how to modify dynamic partitions. "system" is also used as an alias for the active system partition. You will likely softbrick your device and have to fix it!!

- Firstly, remove the strange partition entries

fastboot delete-logical-partition system
fastboot delete-logical-partition system_b-cow

- Next, re-create the missing entries

fastboot create-logical-partition system_a 1000
fastboot create-logical-partition system_ext_a 1000
fastboot create-logical-partition product_a 1000
fastboot create-logical-partition vendor_a 1000
fastboot create-logical-partition odm_a 1000

Note: 1000 will set the partition size to a kilobyte. This doesn't matter as flashing will automatically resize them.

- After that, you should be able to flash to the newly created partitions

fastboot flash system_a system.img
fastboot flash system_ext_a system_ext.img
fastboot flash product_a product.img
fastboot flash vendor_a vendor.img
fastboot flash odm_a odm.img

Note: If you get an error saying "not enough space to resize partition", try

fastboot erase system_b
fastboot resize-logical-partition system_b 1000

- Finally, set the active slot to A and reboot

fastboot set_active a
fastboot reboot

Note: If it doesn't boot or the system starts crashing, you may have to flash the other firmware images as well or try using slot B.

In case this doesn't work, you can ask someone with a rooted NX669J to dump their super partition and send it to you. If you do, make sure to check it first using lpdump. Flashing a corrupted image is going to make it worse 100% guaranteed.

I hope this gives you a better idea of what was wrong and how to deal with similar issues in the future. Of course, a low-level unbrick tool works fine too - but where's the fun in that?

P. S: I'm actually typing this on a flat S7 - still an amazing device, 6 years after release : )