External SSD won't boot after Monterey update

Devices / OS (related to issue):

  • MacBook Pro 15″ 2018

  • Samsung T5 Portable SSD 1TB

    - GUID Partition Map Scheme / APFS

  • macOS Monterey (12.2)

I've been using Samsung T5 SSD (connected via original USB-C cable directly into Thunderbolt 3 port) as my preferred startup disk for a few months now, with everything previously on Macintosh HD cloned to it. The internal HD is already updated with Monterey and working just fine. Wanted to also update software on external SSD to Monterey, and went through normal process to do so.

Update appeared to be installed correctly, for the most part, although took a bit longer than it probably should have. Progress bars advanced and remaining time was given throughout process, but once update seemingly finished and MacBook restarted — for what I assumed to be the last time before finally booting up with new OS — the progress bar under the Apple logo (with no remaining time indicated underneath) made it about halfway through before fans kicked on. Few minutes went by with fans running and progress bar stuck, then suddenly fans surged to loud revving sound and MacBook immediately shut down then restarted itself.

Numerous further attempts have been made to boot using external SSD, with the exact same result occurring each time. MacBook still boots perfectly fine using internal HD as startup disk and, very surprisingly, I'm then still able to access and fully view the SSD this way. It's showing up as a readable/writable device, its info shown in Disk Utility confirms it as bootable, I've been able to run 'First Aid' on it a couple of times (no errors ever found), and it still has plenty of space left (only 460GB used). I'm even able to eject it and plug back in, with it immediately showing up as connected device.

Most interestingly, when I view it under System Preferences → Startup Disk, it's shown as indeed already running on macOS 12.2.

I did make sure to boot internal HD using Recovery Mode, went to Startup Security Utility and under 'Allowed Boot Media' enabled Allow booting from external or removable media, along with 'Secure Boot' being set to Medium Security.

In addition, 'Firmware password protection' is turned off. However, both 'FileVault' and 'Firewall' are turned on under System Preferences → Security & Privacy; not exactly sure if either would play a significant role in causing the issue. The external SSD's 'Security Mode' is also turned off with no password required.

Just cannot seem to figure out what the problem is here and why MacBook won't boot now from external SSD without dragging out time on startup, kicking fans into overdrive, then crashing MacBook and causing it to restart. I've tried altering a few of the settings mentioned above, tried using different ports and cables, tried multiple times to reinstall the OS update on the SSD, etc.  I've even downloaded the Samsung Portable SSD application from their website and have been making sure that it's software is up to date as well.

I called Samsung Support and they were utterly useless beyond belief. Also called Apple and they tried being as helpful and insightful as possible, but the main focus has been reinstalling Monterey on the external SSD through Recovery Mode for like, the fourth or fifth time now.

Is there something I'm missing? Any additional settings I need to change / adjust? Are there some kernel caches or .config files or anything that I could and should delete?

Perhaps a corrupted file or application on the Samsung T5 is causing this? Though, I did frequently run system cleanups, malware checks and app updates using CleanMyMac X right up until trying to update macOS, so I'd be pretty surprised if this were the case.

Any and all input on this would be greatly appreciated. Thank you so, so very much in advance. Cheers.