“It will be important to know if the issue can be replicated in Safe Mode, or not.”
Please trying opening QuickTime Player while in Safe Mode. Try playing one or more known-to-work video files. Does it behave any different in Safe Mode?
If your Mac has an issue that goes away when you start up in safe mode, you might be able to isolate the cause this way.
If your Mac has an issue that doesn’t go away when you start up in safe mode, then the cause is probably unrelated to (1) startup disk functional integrity, (2) optional/3rd party kernel extensions, (3) startup items, (4) user-installed fonts, (5) caches. Instead, focus on other causes.
If your Mac has an issue that doesn’t go away when you start up in safe mode, then we have to investigate further. Only then test this issue in another user account on your Mac.