Looking to take a fixed screenshot at intervals while changing the page in safari for a book
You can do this with the new screenshot key command -  Cmd ⌘   Shift ⇧   5  set to 'Capture Selected Portion'.

Once set, it will capture nominally to Desktop, each titled ScreenShot date/time [eg ScreenShot 2021-03-15 at 4.42.15 pm, so they will file alphabetically. You can capture using Return/Enter or by clicking the Capture button. The selection area is remembered between snaps.
You could potentially automate this, but it's a swift process, so I doubt you'd really need to.
If you need to change the file save location, see How do I change the location of macOS screen captures?