@Marina ******
that is a covid extension you can tell because of the writing on it to the left above your thumb that says ข้อ 5 😛
that part about keeping your stay means IF you want to leave and come back DURING the validity of your stamp you get a re-entry permit <- it doesn't ADD any time to your current stamp. SO you don't want one.
You just need to exit re-enter, and your idea of flying out and back to get a new 90 day entry stamp should be fine