At last.
Picking up from last night; the watch paired to the iPhone 8, and updated to watchOS 6.3. The watch then promptly asked for an update to watchOS 8.8.1, so I let that go. I have no idea when it finally copied over to the watch, as I went to bed with it showing 6 hours remaining.
This morning, I told the old phone to go ahead with the install, which it did. So, the watch is now on 8.8.1.
I brought it with me to work, laid it on a charger, and took out my iPhone 13. The phone detected the watch, paired, and prompted me to install watchOS 10.0.1, so I let it do its thing. Took around 2 hours to download the 2.2 GB (!) file to the watch, then the install.
After the install, pairing continued....and here we are. It only took 3 different iPhones, 2 hours on the phone, and I can now cancel the store appointment for this Sunday.
Oh, observation; the wifi network here at work is a 2.4/5 ghz network. One of the caveats from a suggestion on this thread and elsewhere was to ensure the phone was on a 2.4 only network in order to pair and update successfully. Interestingly, the watch had no issues going from 8.8.1 to 10.0.1 on this network; however the watch would not update at all prior to last night with my iPhone 13 no matter what kind of wifi I was on.
Also, I don't know the age of this battery in the replacement watch, but the health was 100%
Carry on everyone.