>the vasconic substrate theory
Wanna hear my opinion of it?
First let's name some facts:
funnily enough, Basque are no more WHG or EEF than surrounding populations – their genetic differentiation from the rest is a result of development since the Iron Age onwards:
https://hal.archives-ouvertes.fr/pasteur-03199264
Now consider the older fringe theory (Dumézil most prominently) that the Basque languages are related to the languages of Caucasus:
http://euskararenjatorria.net/wp-content/uploads/2017/03/Euskaro_Caucasian_2_pp.pdf
Well, I'm personally inclined to believe that Basque are actually a language from the northern foothills of Caucasus (Circassia), that was carried into Europe alongside their neighbouring Indo-European; that is, that the Yamnaya and Corded Ware cultures were, in fact, NOT monolingual, and that proto-Basque was one of the languages that entered Europe this way.
This is similar to how Magyars invaded Europe, alongside other, mostly Turkic-speaking groups, despite their language not being Turkic they were part of the same migration.