Instagram does not load stories iPhone 8: Instagram has problems loading stories. The circle appears and it is loading dozens of seconds each video or freezing at loading.
What I have done so far:

tested on Wi-Fi
tested on cellular data
turned on/off assistant (to smart switch between Wi-Fi and cellular data)
reinstalled instagram app
rebooted phone

Other apps are working ok, the problem exists only in instagram.
EDIT: Seems like from 2020-12-18 the problem is gone on iOS 14.2. I suspect Instagram app update has resolved the problem. iOS 14.3 seems fine also.
I see that you have already tried a lot of things. Do you happen to have iOS 14.1 ? I heard that this problem happens on it. Perhaps one of these methods will help you:

Clear your Instagram App Cache
Update your Instagram to the Latest
Turn Off Power Saving Mode
Turn off/on airplane mode
Try to log out/log in