Safari Tabs Vanished on MacBook

I see now this is a common problem, but most seem to address other devices.

I had a lot of tabs open in Safari a couple years ago like a to-do list I'd like to eventually get around to. I started using another browser in the meantime and have done system updates, even changing OS, and any time I opened Safari those tabs were still there.. until today!

I don't like to keep it open long to avoid added strain on my MacBook, but I needed to open it a lot lately, including multiple times today. Today, it kept opening a blank tab, but I could get my windows back by opening last closed window 3 times. Opening all windows from the last session together wasn't an option. This was fine until one time it didn't work.

I was able to recover 2 windows, but not the the 3rd - the one with the most tabs. Keyboard commands don't work. Checking the history doesn't show anything. I tried to restore from Time Machine, but it just opened the same as what current opens. I now learned I can bookmark them all to be safe, but how can I get these missing tabs back?

This happened recently with my other browser too and I discovered there's a trick where you click next to the tabs and you can keep opening recently closed tabs as far back as you want, even farther than allowed through the menu. Does Safari have something similar? Why are they vanishing like they never existed?