Congratulations on surprising us all with the v12 Beta. No shortage of goodies for us to enjoy. The vertical tabs support is a welcome addition. Can I suggest you consider adding buttons, or a submenu, to the default Folder Tabs menu as an alternative to going through Preferences.
Not sure about changing the setting often but at least during the process of learning and testing new v12 features I can see myself fiddling with this some more.
The best of both worlds would be a default global setting via Preferences (as now) and a per lister option that could be triggered by an internal command - e.g. Set TABS=left or Set TABS=below.
I'm not sure it makes sense to duplicate that kind of Preferences settings into the menus, except for things you need to quickly toggle on the fly. Otherwise there would be no reason to have Preferences at all and we have everything in there in the menus.
Folder tab position has always been (only) a Preferences setting, with above and below options. Now left and right options have been added, but otherwise it's the same as before, in terms of where you go to configure it.
If we had an option to change it directly, via shortcut, i suppose users would change it more often. Like we aleready can do with Set DUAL TOGGLELAYOUT to change vertical/horizontal.