It sounds frustrating to experience such a drastic difference in battery performance between your Tab S6 and Tab S8 Ultra. Since you've already tried a factory reset and ensured your software is up to date, it could be a hardware issue. However, before jumping to conclusions, trying turning off Wi-Fi overnight as you mentioned could help identify if it's related to network activity. If the issue persists, contacting Samsung support or seeking assistance from online forums where other users might have experienced similar problems could provide more insight and potential solutions.