I agree. I’m about to switch from a Windows PC to a Mac (a MacBook Pro). I’ve watched a lot of YouTube videos about MacOS to prepare myself and I think that the thing that’s going to bug me most is the lack of proper vertical tabs in Safari.
I always prefer to use built in apps if they are good enough and IMO Microsoft Edge does vertical tabs really well. Like you say, Safari’s sidebar is almost there, but as well as the issue you highlight (not being able to hide the horizontal tab bar) as far as I can tell it also doesn’t have the feature that Edge has where the sidebar (vertical tab bar) can be set to icon only so it only takes up a tiny bit of space on the left of the browser window but when you move the mouse pointer over it, it expands so that you can see the first part of the URLs for each tab. That setup works perfectly for me because I can mostly tell what the open tabs are just from the favicons visible when the sidebar is collapsed but when I mouse over to it to actually switch to a different tab I can see the actual URL to be sure I am switching to the tab I want. I really wish that Safari would implement that feature for its sidebar.