Oh, silly me!. I forgot to mention the
back door to Spain. It's called "
Portugal"...
Portugal was one of the earliest allies of England from the 1600s or earlier, then the United Kingdom. And helped out with the corsican gent in the 1800s most notably. In WW2 a bit frosty relations as the Salazar(?) regieme was a dictatorship, but still, despite being kinda-sorta pally with Franco, they could flip over to the Allied side.
So Spain would have at a minimum to divert a lot of divisions to "observe" the Portugese border, which Portugal just might object to. Divisions that
wont be of any use to Adolf and his team. Even if they stay sorta neutral, these would be required "just in case" they actually did flip to the Allied cause.
Should Portugal actuallly "flip" Allied then that opens up
uncontested ports and land area unopposed for the Allies to deploy into and backstab the Spanish from. And air bases already set up for use etc. No need of an OVERLORD over the beach type assault landing in Spain proper in that case!.
And the Allies could provide all sorts of enticements (loadsamoney being a big part of those!

)to encourage the flip to overtly on side with them methinks. They managed to get leases on the air bases in the
Azores in our tmeline already.
https://en.wikipedia.org/wiki/Portug..._War_II#Azores.
I had a unit of Portugese cacadores in my 25mm Napoleonic British army back when I was 16 and interested in Horse and Musket wargaming. Almost as good as the UK rifle regiments, just in brown coats and a bit cheaper in points, as "B" class rather than "A", IIRC.