Can't launch Startup Security Utility
As I'm sure you know, troubleshooting is a process of elimination and often requires patience. 

The first thing I would do is remove any non-Apple hardware (if connected) and try again. If this makes no difference, then I would proceed with resetting the NVRAM of your iMac Pro.

How to reset NVRAM on an iMac Pro

Follow these steps:


If you have a firmware password, first disable it before proceeding
Fully shut down your iMac Pro
Switch on your iMac Pro
Immediately press and hold the OptionCommandPR keys. 
Keep the keys down until after the Apple logo appears and disappears for the second time. 
Allow your iMac Pro to finish booting up. (NOTE: Upon reboot you may need to reconfigure some of your System Preferences (e.g. sound volume, display resolution, startup disk selection, time zone, etc).
Now reboot your iMac Pro and use the Command + R keys to boot into Recovery Mode and see if you can get the Startup Security Utility to work.


If this works, all good! If not, let's investigate Single User Mode again.

Single User Mode

Since your iMac Pro itself seems to be running fine in normal usage, another thing to investigate is the issue you're having booting into Single User Mode. Some additional things to check:


If you have a firmware password you must disable it first. You can't boot into Single User Mode (or Verbose Mode either) with a firmware password enabled. So, if you have one, disable it and try again (I suspect if you had one it'd be disabled anyway if you followed my steps for resetting the NVRAM, but just wanted to be thorough and mention it again).
If you didn't have a firmware password, or if disabling it doesn't work, then try booting into Verbose Mode instead. The process is the same, except that you're holding down the command + V keys instead. Verbose Mode is actually a normal boot, except that you're seeing a description on screen as to what's actually happening during the boot process. You'll know you're in verbose mode when you start seeing white text on a black screen. 


If you can boot into Single User Mode, great! If not, let us know if you see anything of interest when booting via Verbose Mode. 

About the AdjustBusy timeout error

The AdjustBusy timeout error while booting into Single User Mode is interesting and one of the reasons I asked at the outset for you to remove any non-Apple hardware. Typically this relates to an IOService (don't worry about the technicalities - it's basically just referring to input/output devices and drivers).

Anyway, if you're still seeing this error, I would do the following:


Boot your iMac Pro normally
Launch the Terminal app (found within your Applications > Utilites folder)
Type (or copy and paste) the following command:


kextstat | grep -v com.apple


Press Enter
Within moments Terminal will list all 3rd party kernel extensions (Note: you may need to stretch the Terminal window so it's easier to read). 


The above process is used to identify any 3rd party extensions you have installed. While this is a list of all 3rd party kernel extensions, I'm interested specifically in anything you don't remember installing, or something that belongs to software you no longer use. Also look out for anything that could be related in some way to LaCie hardware (either current or old) or to SoftRaid.

Let me know if there's anything of interest.