It did take them 13 years to make the next mainline game. Spinoffs don't count. I love Kingdom Hearts, but SE really dragged this whole saga out way too long, and I'm still mad at them for it. And I don't hate the spinoffs either. 358/2 Days holds a special place for me. But damn, KH2->KH3 13 years? Come on SE. Not cool.
None of the games are spinoffs though.
Unless you mean filler which in this case the only game that is filler is Re:Coded.
BBS and 3D were mentioned by Nomura as mainline titles specifically during interviews.
Well I disagree with Nomura. There's a reason KH3 has a 3 at the end.