The 16GB card should be ok to use, generally up to 32GB sized cards of most brands can be read by Garmin devices for flashing and even larger cards, i.e. 64GB> provided they're reformatted from exFAT to FAT32. I've had less problems overall using small cards when flashing however.

The uncontrolled boot-looping is another troubling sign TBH. Holding the power button down until the screen goes black should not result in it re-starting spontaneously, it should be hard-off. Regardless, let's press on.

Your suggested sequence is exactly what you need to do. With boot-looping devices i've sometimes had to use my elbow to hit the Enter key, i know exactly that feeling of needing a third hand to click OK fast enough on the GUI.

Even boot-looping devices that can be shut down completely are tricky because they will immediately start looping again when powered on either by the button or a 5V USB connection. Spontaneously powering on adds usually more workload than 2 hands can handle easily so practice the keyboard elbow method. Be persistent because it might take numerous attempts to catch the right moment but provided it's a software problem, once data transmission commences from Updater.exe to the device the boot process will be suspended and the flash will finish without interruption. Unfortunately flashing will always fail if it's a hardware fault causing such behavior. Good luck.

PS: Almost forgot about this:
Do I need another power cycle to execute the new boot.bin and in turn, the region bins in the SD card?

If you believe the "hung" device is waiting to mangle the SD card as soon as it sees it, what can I do? That "device available" window is literally just a few seconds - if I insert the card then, I'll have no chance to start updater .....
If the flash starts and isn't interrupted each command will be executed in turn and if one cmd fails the next will be attempted. However if the flash starts and then fails before finishing there may be a "last_id.bin" file written to the 1947 folder on the card. If so you must remove that BIN before another flash or it's presence will not allow another attempt. If you mean do you need to start from scratch on each flash attempt, then yes. The device must be in a 'power-off' state at least momentarily before each attempt. Your chances of catching it between boot-loops are slim to none so powering it off by holding the button down and then quickly connecting the USB gives you a decent chance. BTW, if the device stays off as long as you hold the button down that'll give you the best chance of all.

There should be no 'mangling' of the card caused by the device boot-looping (i guess you mean causing corruption).