I was experiencing similar issues. Even after a factory reset of my Watch and a factory reset of my iPhone (with a subsequent restore from iCloud) I was running into the issue of the Watch app not launching or presenting me with a syncing screen. I eventually was able to work around it by removing the Watch app from my phone. As many might discover, starting in iOS14 you can not actually "delete" the Watch app, rather you can "Remove it from your home screen". I found that by going into Settings-->iPhone Storage-->Watch and selecting the option "Offload App" I was actually able to remove the app off my iPhone (I had a hard time finding the Watch app in iPhone Storage so I actually used the magnifying glass in the upper right of the screen to search for it). Once that was accomplished I was able to go into the App Store and re-download the Watch app, to my astonishment the Watch app launched and functioned without issue.
A CAUTIONARY NOTE: After re-downloading the app and "setting up my Watch" I noticed that while I was able to setup my Watch from my prior backup I had lost ALL of my activity and health data. While somewhat frustrating I felt it an acceptable trade-off since I was able to use my Watch and iPhone again.
Not sure if it will work for others, but I figured I would share my experience with others. Good luck.