Just to be sure you want to side-load it to your watch not upload it to the store correct?
First off have you rebooted (not turned off and back on) your computer after generating the new Developer Certificate. Have you deleted the .tpk files and regenerated them before rebooting? Just need to be sure there are no caching errors. It really sounds like a caching problem.
In the USER\GalaxyWatchDesigner\Keystore folder look for your author.p12 and distributor.p12 and make sure those are indeed the correct dates.
Is the OPR for your always on display below 12% I know the requirements are 15% but sometimes they won’t install when it is over 12%
On your watch press and hold the home key until it says rebooting takes about 15 seconds.
Restart Computer, Phone as well
Rebuild your watch face again
When you go to push it to the watch you will need to OK the RSA encryption key again as rebooting forgets your acknowledgement.
If it still doesn’t load can you create a support request and submit the .gwd file and mark it for Ron and I’ll see if it loads on my watch.
Samsung Developer Program