Aside from what you've tried, I've also seen Reminders be an issue for some, particularly if you attach a lot of files or images to your reminders.
I'm posting this as my Series 3 storage issue doesn't seem to be related to iCloud, but to Siri instead. There seems to be an widespread issue affecting various Apple Watch models and dating back to WatchOS 6 in that the local Siri voice pack (~0.5GB in size) doesn't correctly download and apply. Predominantly this seems related to people using the British Female voice. If you have this issue the most obvious symptom is that when you hold two fingers to the dial the Watch will speak the time in the more robotic VoiceOver voice instead of Siri. If so your Watch will be continuously trying to download that voice pack whenever it is put on charge, fail, delete the download, and then start all over again. Although it deletes the failed download I've noticed that over time the free space decreases, as though it's not deleting all of it. On a series 3 this is more obvious as we don't have that much free space to begin with!
If you think this is affecting you (try holding two fingers to the dial to tell) I'd recommend first turning Siri off on your Watch by going to Settings on the Watch, and disabling the three Siri options ('Hey Siri', 'Raise to speak', and 'Digital Crown'), turn the Watch off, back on again, re-enable Siri, and then following dave-porter's post here... https://discussions.apple.com/thread/251845629?answerId=254126905022#254126905022. Although it didn't work all the way for me (I got it to work with the American voice, but not back to British), it might be worth a try.