Yes it would be great to minimize the sidebar to some 2-3px width, pretty much like in Vivaldi, or how it was in the old Opera (Presto).
Vivaldi has "status bar" while Opera doesn't, so with your suggestion there would always have to be some 2px graphic indicator in the side to reveal the sidebar.
Workaround for you (at least for time being): Set custom zoom sizes for pages, even for speed dial, set it to 90% and leave it and it will be like that until you reset/change it.
If you know css, you can do a local modification for the sidebar to set it's width and size issues. I don't remember the exact location but if ur interested do this. Open start page, open dev tools (ctrl + shift + i) and examine reborn navbar css stuff. You can''t change them all though but it's possible to play with width of it.
