Jad, we have exactly the same situation. My wife's S5 stopped updating after version 10.nn and even though its battery health is in the low 80's it lasts all day and evening every day. My S6 has gotten better since 11.1 and 11.2 The only thing I do differently from others here with 11.2 is that I do not have Passcode turned on (makes the watch way less annoying, too!) I have optimized charging turned on (only charges to 80% overnight to preserve battery life...my battery health is 93%). I also have handwashing, mindfulness, all the music and podcast settings off, and I don't use Apple Pay. From 7 or 8 am to 11 or so pm, my watch goes from 80% down to 35-40%.
I really want my wife to upgrade to a new watch from her S5, but she says not until the battery problem is solved.