Hi. I have a Poco F2 and a Mi 10 Lite on MIUI 12. You should have the same on the 8.
I can toggle the Ultra Battery Saver to On using the Control Panel pull down from the upper right side of the screen. If it is not visible you can use the pencil / edit icon to drag it to the main Control Panel toggles.
However, looking at my Settings - Battery & Performance you should have seen the Ultra setting there. What MIUI are you on?