Don’t find much use for them personally but some are in the tabs frame of mind and some are in the buffers, whatever works for you… I just think in buffers and the buffer list. Maybe if I have a very separate set of work going on I might want to put it in a tab. I think of tabs in Emacs more like a workspace or session (ala Tmux)
Syncing tasks app with Thunderbird using CalDAV rather than your json method is one way of doing it. There may be other tasks apps out there though?..You’d need a CalDAV service for hosting the tasks…I use Posteo (email/calendar) and it supports this.