Safari opens a page with many tabs and does not respond.

Quit Safari first. If it is unresponsive force Safari to quit: How to force an app to quit on your Mac - Apple Support.

Then, press and hold a Shift key while launching Safari again from Applications or the Dock.

Release the Shift key when Safari opens.

I understand that you were able to open Safari by using the Shift key technique, and everything appeared to be working properly, but the next time you opened Safari normally it was overrun with tabs.

Do any of those tabs appear to be sites that you intentionally loaded? In other words are they being populated from your previous browsing history, or are they arising from some unknown reason.

Try the steps described here: If Safari doesn't open a page or work as expected on your Mac - Apple Support. Some of those suggestions are more useful than others; for example, Safari Extensions could be affecting its operation.

This is exactly what I am seeing. Did you ever get it sorted? Every time I restart Safari thousands of tabs are open and Safari is OBVIOUSLY almost non-responsive.