Hello Grayerbeard. If I understood correctly, the YYYY-MM-DD date format you want to change is located in the tags/properties or task plugin of your page? and your Windows 11 is using the correct DD/MM/YYYY format like the 8th point of this tutorial shows?
If yes to both questions, maybe you could find futher information on the end of this thread, on which an Obsidian team Moderator tries to determine if the problem is coming from Obsidian or Windows itself:
This is all I can offer on this issue, as Pch’s message is well documented for the rest.