Tab or Session Manager for Safari 12? Safari 12 improved the security of the browser by killing off the older API. Unfortunately in doing so it killed the very handy Sessions: https://sessions-extension.github.io/Sessions/ 

I can easily go find the tabs I had open when I upgraded. 

My challenge - where do I find a robust tool that remembers all of your tabs even after a crash? 

(Yes I know of the preference "reopen open all windows from last session" - past experience says that this doesn't work if the browser crashes).
I have had the same question and found no answer. So I myself developed a Safari extension named Tab Space which could handle this task for me. Try it if you want.