**Potential Solution**
I had same problem with 15 Pro and Watch 5 - I began when transferring to new phone, prompts kept appearing to connect watch, process began then froze and watched was bricked.
I experienced it all - can’t connect for 8 hours, tried hard reset, network reset etc. no joy.
I solved it by using old phone to fully unpair watch, reset and full reset/ wiping the watch.
Then doing the full off, on of both new phone and watch.
The watch app on the phone then recognised the watch as ‘new’. It proceeded to update OS on the watch. That took 2 hours! Failed. Then another hour. Watch still froze once, so needed to repeat the process. Then froze when setting up Apple Pay. Finally, It took a long time to pair and update - like 45 minutes. But eventually (after skipping Apple Pay during set-up) my old watch is back with all apps etc (despite setting up as new and despite a full reset)… thankfully.
So solution is basically a full hard reset, a bit of luck and most importantly, patience.
Side note: Pro 15 - amazing camera. Good battery and charge but awful setup experience. Overheating, passwords not transferring (work on old phone, iPad and Macbook but not new phone 🤷🏼♂️) and all-in-all around 7 hours to get set up.