Hey,
I'm experiencing a very similar issue as well. I have a series 5 cellular and I normally end the day on maybe 85% with light usage and yesterday I updated and ended the day on around 60%. Currently it is the following day and I'm at 45% where I'd normally be at 70%. WatchOS 7.3 is definitely causing this issue and I have no idea how to resolve the issue. Anyone have any ideas other than wait for a software fix?