"I have a iMac 2019 (yeeeukk T2!!) which has stalled on OS load for reasons unknown. "
2019 iMacs (other than the iMac Pro) DO NOT HAVE a t2 chip inside.
I have your "pathway forward".
Whether you choose to follow it... is up to you.
Here's what you need to do:
a. Get an EXTERNAL DRIVE. It can be either platter-based or an SSD. An SSD will work MUCH better.
b. Once you have that, connect it to the 2016 Macbook and open Disk Utility. ERASE the external drive to APFS, GUID partition format.
c. Take the external drive to the iMac.
d. POWER OFF the iMac and connect the external drive
e. Press the power-on button and IMMEDIATELY hold down this key combination:
Shift-OPTION-Command-R
This should invoke the OS installer for the version of the OS that came with the iMac (High Sierra or Mojave, I think).
f. Do the OS utilities load?
If so, this was an important step. Do this next:
g. Open the OS installer and begin "clicking through". HOWEVER....
h. When the OS installer asks WHERE you want to install, "aim it" AT THE EXTERNAL DRIVE. Then let it go.
i. This will involve one or more reboots, the screen may go black for a little, and it will take time. BE PATIENT.
j. When done, you should see the initial setup screen (choose your language). Set up a basic account on the external drive. DO NOT BOTHER with iCloud, email, or anything like that. All you want is a username and password.
IF YOU CAN GET THIS FAR, you now will have a bootable external drive with either High Sierra or Mojave on it. IF YOU CAN GET THIS FAR, you should now be able to "go to work" on the iMac's internal drive and get it right again.
This is your pathway forward.
PRINT THIS OUT and check it off as you go along.