Thank you for providing this detail, I used to use tree tabs before the webextensions fiasco but hadn't gone back to it yet as I wasn't aware it was possible to hide the existing tab bar. I'll definitely give it another shot now thanks to your help!

The last time I tried TST, it was fairly hostile to my battery on macOS; is that still a problem?

Sidebar tabs are a massive workflow upgrade, it surprises me that it's not a core browser feature, especially now with the ridiculous proliferation of those loathsome vertical-space hostile sticky elements that have spread through the web like a plague.