I actually prefer this approach, but I think it would be more useful to leave the syncing of the emails to a different program, and then just managing the labels / tags with your tool (which is notmuch territory). So the workflow would be:

Initial or download (only remote changes):

1) Download email ("All Mail" folder) with whatever you are using (offlinemail, ...)
2) call your script to index the downloaded emails (All Mail" folder) with notmuch and get the labels and set / remove them in notmuch accordingly.

Upload (only local changes):

1) upload emails using whatever (offlineimap, ...)
2) upload the changed tags and create the needed new labels (and possibly delete empty ons?)

The tricky part will be the synching when changes on both sides were observed.
The mail sync is implemented in e.g. offlineimap and works reilably. But I don't know how one could handle tag / label changes on both sides.

So, implementing the mail sync in this script would, as I see it, kind of reinventing the wheel. One can leave it in, but I would like to have an option to leave the synching of the emails to a separate program.

Cheers,

Rainer