“a critical software update is required for your Mac, but an error was encountered while install this update”

I hope this info is useful to someone. I used the mac with the above workaround for a while but I have since installed OpenCore Legacy Patcher to install Sonoma on this unsupported 2017 MBP.

The patcher initially didn't enable the touch bar but after giving it some time to install the post install patches the machine runs fine. Touch Bar works as it should and so does the touch ID. So there was no hardware failure to cause the initial issue. I have since read that Apple servers are fussy about the DHCP used to access them. Someone else suggested using googles DHCP addresses to get through the critical software update issue. Hope this helps, and best of luck if you have this issue.

Just a follow up on the above. I used the mac fine for an hour or two then I shut the MBP down and was back to square one with the "critical software update is required" (sleep work fine) . I guess this is why it is a "temporary" fix.

Anyway I when back and followed the instructions to renew the DHCP. I remembered that after restarting following the DHCP renew I was presented with the same screen to select a network for downloading an update as before. I selected it and it returned with the same "critical software update is required" presenting me with Shutdown or try again. I pressed the try again button and on the third try it worked! This also happened on the first successful startup I'd just forgotten.

So be persistent and once you get there don't shut down your computer!

Cheers