Hmm, regarding using GRE. You could, but should you?
Again, what you have now, if you have partitioned nonzero areas, such should function as different nonzero areas.
If you combine partitioned areas, you could stress your network with a too large area, a primary reason we use multiple areas!
Second, using (specifically) GRE, you might create MTU fragmentation, between ABRs. Remember, an intra area path takes precedence over an inter area path.
At the moment, possibly the biggest issue you have is your OSPF design is "unusual", because of the intentional nonzero partitioned areas.
Possibly, the best approach is to consider whether your independent areas should follow the norm and have different area numbers.
I didn't post it, but I also changed, in my lab, one partitioned area 6 to area 66. LSDB information much like my first LSDB info, but perhaps a little clearer.
What you may want to further consider is whether you may eventually do some of the things areas support, differently, for specific areas. If so, then you'll likely want different area numbers.
