Poly is great, Disney is magical, and you will have a great time. So special to have grandparents going as well!
That being said, I really would consider 3 rooms at a moderate over 2 rooms at Poly. I know I would choose that without question if I had the same travel party as you. As magical is Disney is, it can also be hot, tiring, crowded, and over stimulating. It can be really welcome to head back to the room to rest, have some space, a good night sleep, etc., and I think that could be tough with 6 people in a studio.
Although, Port Orleans is bus access only (other than boat to Disney Springs), I think it is a beautiful and relaxing resort. Honestly I may even prefer it to some deluxes. Plus it is fairly centrally located, so even though you aren’t right at any one park, none of them feel that far away. I also have had generally good luck with the buses there.
Good luck with your decision and have a wonderful trip!