OnePlus 5 Soft Brick
This will be a good workaround for the situation above. Since you were able to flash the phone, that means the MSNFlashTool is well configured in your computer. Now what I want you to think about is how you can flash a custom recovery image into the phone so that you can even get the phone alive by a third party OS like Lineage and anything. 

After downloading the firmware file and starting flashing, the MSMTool writes the current progress in the progress window writing what it's flashing in the phone. Notably the userdata.img is one of the files you recognize since it is somehow big and doesn't flash quickly. 

Consequently many data partitions exist in the phones memory, like the:


system.img
cache.img
boot.img
recovery.img


These are the essentials in any system, remember these are not all but these are the compulsory in any system. I'm sure the flash file is an archive that you can open with rar. It sometimes can be an archive containing an archive so you have to extract it twice incase you don't see these img files. Once opened in rar take note on what type of archive is the firmware file. It can be:


zip
tar
7z
rar
gz
gzip


And many more other types. Hope you remember this. Look for a compatible custom recovery image like twrp and replace it with the recovery.img or alternatively the boot.img if you miss the recovery in the firmware. If there are checksums like the md5, sha256 or any other sums to verify the integrity of files, regenerate and replace them, then repack the firmware with the same compression type as it was and flash the phone using it and you shall have teamwin recovery as your default recovery or boot. 

Use the recovery to load third party OS and if you get the OS booting use it to unlock bootloader for futher fastboot actions or you can even use the new OS if you're convenient with it

Note: Ensure the recovery image is specifically made for your device model. If you have any trouble finding the appropriate recovery for your specific phone model, I can make you a download link for it after you give me exact specifications of your phone. 

Hope it helps you