I did exactly as you said in your post above, with one addition.
I did the update in another room where I could not watch it on my iPhone.
So I was not tempted to watch how slow it was.
I restarted both my watch and my iPhone XR.
Then put watch on charger, started the download and
then left the room and worked on my iMac in a different room and forgot about it.
When I first looked at it , it was installing the download on my watch.
Next time I looked at it, it was done.
It seemed fast to me. But it actually took about 90 minutes or more. But I didn't notice it
Apple instructions say you MUST use an iPhone XS or later.
Apple is WRONG.
I used my iPhone XR.