Your solution to put all the tunnel interfaces into area 0 for the interconnections could potentially work. You do want to be careful then that area zero is able to connect to and see all other areas and there are no isolated area 0 interfaces because of your topology. Typically as you noted, we have a hub site that is in area 0 with all the tunnels then connected there to insure area 0 integrity. The other option is to just put everything into area 0. If you have less than 50 sites there should be no problem with the database size or the ability of the devices to keep track of updates. This would be simpliest if the site count and number of routes can be handled.
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