How can I get Edge to restore all the tabs I had open last session?

I always check before closing so this answer is useless.

You can copy open session files via Shadow copy service / VSS. Hobocopy isn't maintained but works... via commandline.

You can even use %LocalAppData%\Microsoft\Edge\User Data\Default\Sessions for a universal path.

Now if those session files only were decodable / interactively browsable to extract just needed parts, instead of being a huge dumb monolithic snapshot. (Technically they are, but no convenient apps are available)

How can I get Edge to restore all the tabs I had open last session?

I suspect that like me, you opened a new window, and not realizing you had closed your main window (or some such) and now you only have the one "new" window even though you have tried relaunching edge.

I don't know why there isn't a menu option for this, but when you click the title bar, or press ctrl+shift+t, it re-opens the last tab in the same window. A second press of this key combo restores your previous window, tabs and all.

Alternately, the sessions are saved (at least on my machine) as:

C:\Users<USERNAME>\AppData\Local\Microsoft\Edge\User Data\Default\Sessions

I've found that making a backup copy of that after closing edge (when edge is running, it prevents you from copying), then when you find that you've accidentally borked your tabs, restoring the backup (without edge running) fixes this. But! You have to backup every time you leave edge. It's difficult to manage.

Missing the point Edge should open with the tabs we leave open not make us play hunt the tabs.