Hello,
I had a similar issue with my apple watch. My issue was with an iphone 10 and an apple watch 3. My apple watch stopped charging on any charger I tried many solutions until I did a full reset. After that pairing became impossible just like you. It sounds eerily similar to your scenario. seems like the reset was the trigger for both of us.
I tried every solution just like you including different phones and nothing works. I think at this point its either the apple store or a new watch.