I have created the following Shortcuts that can be activated with F9 and F7.
Forward By 10 seconds https://www.icloud.com/shortcuts/ebd6b399c5154367b49124d313aef1ba
Backward By 10 seconds https://www.icloud.com/shortcuts/02cf0870552c43dda69d4a7105c62be8
Just double check that "Pin in Menu Bar" is checked and "Run with" have the desired keyboard shortcut. This way they will work even if the Shortcuts app is not active.
