Stop video autoplay in Safari Reader Mode

Please try changing customization options for websites.

Click Safari in the Safari menu in the Safari menu bar and choose "Allow All Auto-Play".

Move mouse pointer to Auto-Play, click inside the dropdown box and choose "Stop Media with Sound ".

Test. https://support.apple.com/guide/safari/customize-settings-per-website-ibrw7f78f7fe/mac

If it is successful, follow the guide in the link posted below for all other sites

https://support.apple.com/guide/safari/customize-settings-per-website-ibrw7f78f7fe/mac

After installing an Update or Upgrade, starting up in SafeMode is not a bad idea.

Startup in Safe Mode. https://support.apple.com/en-gb/HT201262

Is there any adware installed?

Test it.  

Remove adware if installed on your Mac..

       Run the latest release of Malwarebytes for Mac to remove malware/adware, if installed on your Mac.

       For instructions: Install Malwarebytes for Mac v4      Uninstall Malwarebytes for Mac

       Click the “FREE DOWNLOAD” button.

       Click the “Scan ” button. Once done, quit Malwarebytes for Mac.

       Restart the computer and relaunch Safari holding the shift key down.

       Scan for Malware again.

I just visited a website with embedded video player.

I cannot remove the player but I can stop it playing.

I clicked the stop/play button and hit the play to stop the video.

Of course Reader view will stop showing the distractions.