FYI, a PRAM reset wipes out the nvram where the pointer to the bootloader resides. Sometimes performing a single PRAM reset is not enough and you need to hold the PRAM reset for a full minute. If you look closely at the laptop's display you should see the backlight come on changing the pure black screen to a slightly lighter black just for an instant. It helps to see the effect if the room is darker. If you don't see the backlight flicker on for an instant about every 20 seconds, then the PRAM reset is not being activated. If the PRAM reset worked, then the laptop will search out and boot from the first viable bootable volume it finds whether internal or external.
Connect a USB stick (or drive) to your other Mac and Erase it, making sure to select "GUID partition" and HFS+ (Journaled). Use Disk Utility or Apple System Profiler to get the Apple identifier for the external drive which should be in the form of "disk1" or "disk2". In my example later, I will use "diskN" for the example template, but you will substitute the correct number for your USB stick/drive in the place of the "N".
We will use the Terminal app to run the rEFInd installer script with the "--usedefault /<path-to-external-SSD-EFI>/" so the rEFInd bootloader gets installed to the external USB drive's EFI partition using a generic form of the bootloader which should allow the Mac to boot. Plus this should not affect the host Mac's nvram settings.
In the Terminal, type "sudo " (without the quotes) followed by dragging the rEFInd installer script (called "refind-install") to the Terminal so it auto fills the path to the script. Then continue typing " --usedefault /dev/diskNs1". Then press Return to execute the command which will prompt you for your user password. Here is an example template of the command:
sudo <path-to-refind-installer>/refind-install --usedefault /dev/diskNs1
If your external USB stick or drive is "disk2" and the location of the rEFInd installer is at "~/Downloads/refind-bin-0.11.4/refind-install", then the command would translate to:
sudo ~/Downloads/refind-bin-0.11.4/refind-install --usedefault /dev/disk2s1
FYI, the "s1" is usually the hidden EFI partition on a bootable Mac drive.
You should make sure to have good verified working backups of your second computer just in case something goes wrong. Unmount the USB stick/drive and connect it to the original laptop to see if it will boot. This will only work if you were getting the blinking folder with question mark before. Give it time to search for bootable volumes as it can take a while. Download the rEFInd binary zip installer from the link on this page. If macOS doesn't automatically unzip the file, then just double-click on it.
If the rEFInd USB drive works, copy or re-download the rEFInd binary zip installer file to this "broken" Mac and run the installer normally as described here so your "broken" laptop will automatically boot rEFInd and be properly configured for that particular laptop. Make sure to remove the rEFInd USB stick/drive before running the rEFInd installer so the USB stick/drive doesn't interfere with the permanent rEFInd installation. The rEFInd bootable USB drive we made was just a generic rescue disk.
If none of the above works and if you haven't done so already, I think you need to completely erase the SSD and start over. To completely erase the SSD, you need to select the physical drive in the left pane of Disk Utility. If the physical drive does not appear, then you need to click on the "View" drop down list just above the left pane and select "Show all devices". Make sure Disk Utility uses the GUID partition and select HFS+ (Journaled).
If you have need files from the original Mojave install, then use your enclosure to connect the SSD to another Mac running HS or Mojave. I've seen someone mention that Paragon may have a Windows APFS drive, but it is not free.
Please let us know how you make out with all of this and Good Luck.
