Exactly the same problem (Series 10). I updated the watchOS on Monday when it came out and noticed it won't charge anymore due to the heat, it discharges instead. Then the battery died few hours later and won't charge again. I couldn't reset nor unpair the watch as the "Watch" app won't even open. (16 Pro Max running iOS 26). When I tried to pair it with my 15 Pro Max (iOS 26) I noticed the OS became very leggy as soon as it's paired, just like my 16 Pro Max on iOS 26 beta since the beginning, perhaps there were some links there? The watch remains unusable, I had to erase and remove it from my iCloud as the Watch app won't even launch.