Or Asturian-Leonese, Galician, and Oliveno are halfway between Castilian and Portuguese, better. Or Catalan, like Occitan, is halfway between Spanish and French, better.
Getting even fancier, Extremaduran-Cantabrian is halfway between Asturian-Leonese and Castilian. Or how about Mirandese and Rio de Onorese, halfway between Asturian and Portuguese. Getting even crazier, how about Murcian and Andalucian, halfway between Mozarabic and Castilian. Or Eonavian-Ibino, halfway between Asturian-Leonese and Galician. Or Fala, halfway between Galician and Asturian-Leonese.
I’m not quite sure what to do with Manchengo and Aragonese. They’re just splits off the Castilian trunk. The latter is for sure a separate language. The former, could be.
The Spanish from the south of Spain is quite different from the Spanish of the north of Spain. Castilian actually came from way up in the north in Cantabria and spread south after 1100, taking out the Kingdoms of Leon, Asturias, Aragon, etc. as it moved south. The Kingdom of Castile is simply the kingdom that came out on top, so it’s language got to be the standard.
Leonese, Asturian, and Aragonese are simply the forms of Iberian Romance that were spoken in those Kingdoms.
Interest fact, Portuguese is actually a dialect of Galician and not the other way around! Not only that but Portuguese and Castilian were one language until Galician-Portuguese split off in 1300. Galician and Portuguese did not split until ~1500. The split between the two is analogous to the split between Scots and English.
Aragonese didn’t split from Castilian until later, I believe in the 1600’s. That’s why it is so close to Spanish. Asturian-Leonese and Castilian probably split ~1300 also.