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.