I had the exact issue on my Series 5 44mm after updating to 7.4.1. I tried unpairing it and re-paring it via the Watch app on my iPhone a couple times. The first time I restored the watch from backup. Since that didn't help, the second time I set it up as a new watch. I kept things super minimal and only installed 6 apps, but it didn't make a difference.
I finally fixed it by doing a Reset on the watch itself by going to Settings -> General -> Reset -> Erase all Content and Settings. After the watch reset, I paired it again and restored from backup. Now my watch is back to having around 60% charge remaining after a full day. In other words, I had to Reset/Erase the watch from the watch itself since unpairing from the Watch app on my iPhone did not help.