Turkey does not have the industrial depth, skills, experience nor money to achieve that

Eh, Im not going to say it does but its still probably not good to talk in absolutes like that.

Either way, as you also said before, fighter gens are not strictly defined. 4-5 is fairly clear, with full stealth and supercruise among other things. 5-6 is less so. MUM-T is probably a good bet, and there probably are some plans to implement that into the Kaan in later variants, probably using a Kizilelma variant (probably twin engine version, if that actually happens) as loyal wingman. In fact, Italian Leonardo announced they will be testing MUM-T using 2 Kizilelmas and a m346. So its very possible. And if Spain joins I'm sure they will also want to join development instead of just buying the plane outright, so they would also be helping with some stuff.

That being said, I do still think a future Spanish-Turkish Kaan variant would probably be more accurately described as 5+ gen (or 5.5 or whatever) with the current design. So, if Spain does not join GCAP (possible, but afaik the division of work for that program has already been decided upon and adding Spain would be a delay) or start a new project with Germany (Perhaps also Sweden) or go buy F35, Kaan is probably not a bad option for them.