You need to be root and running preferences manager app. And oneplus camera version 3.10.17
Here is what you need to do:
Open preferences manager app and search for camera 0 and 5 xmls.
On each xml you hit the plus button and add a new string set called Video960FpsSizes.
On the value you write 1280x720
Save both xmls and you are done! Now a third 960 fps option will appear on slow motion mode
Shoutout to @docnok63 for his initial research and ideas.
If anyone adds it to a magisk mode or patch, dont forget about us on a small mention just out of cortesy