My wife has Series 3 and I have Series 5, but I haven't seen a major difference between the two. Series 5 is slightly sleeker and smaller, but the updates didn't feel important enough for her to upgrade.
I know sometimes something in the software update can glitch and cause aggressive battery consumption. I wonder if you were affected by something like that (which is something they should address to prevent from happening in the future).
https://www.apple.com/watch/battery/#:~:text=Apple%20Watch%2....
If you use your Watch more passively, all-day performance is not out of the question. It all depends on your individual use case.
Besides, 18 hours should be sufficient since you ought to be getting more than 6 hours of sleep a night anyway. Somewhere in that offtime is when you can charge. I do it overnight, but plenty of people charge during their morning routine (shower, etc.) and say it charges more than enough to last all day.
My expectation is each time the chip will be more efficient, but they can choose to use that on a brighter screen/more features, or better battery life.