I've tried simple tab groups before but it was occasionally a bit janky and I was looking for a different UI implementation.
Not really keen on using a sidebar either and the majority of extensions I found all seemed to be using the sidebar.
+1 for tree style tabs. Every time you open a link it is automatically a child of the current tab, so you don't have to manually group everything. (Although you can)
Makes me hate the tab groups in dege/chrome every time (work ☹️) I'm forced to use them.
Tiled Tab Groups is similar to Simple Tab Groups, but better imo in some details.
You're looking for something limilar to tab stacking, but said you don't want to use sidebar... But the closest to tab stacking is using an extension that organizes your tabs in collapsable tree structures, like Tree Style Tabs and Sidebery.
I use is Sidebery because I like vertical tree tabs and Sidebery is better than using Simple/Tiled Tab Groups + Tree Style Tab, I know because I spent a couple of years using this combination before switching to Sidebery.
Screenshot of my Firefox. You can see I have four groups with 110+5+68+82 tabs. There are tree pinned tabs visible no matter what is the current active group. Then there's the list of tabs below.