Looking at OP, don't take Burgos straight away. If you force all the Nationalists into the North West corner of Spain, you will have to fight WWI to finally kill them. Best to surround the capital and take it last.
Yeah, that's my usual strategy. Doesn't always work out that way though

I think what went wrong here is that the enemy capital was Burgos (historically inaccurate I think? Pretty sure in reality it was Seville), and I wasn't able to hold Euskadi. Once I lost control of Santander, I don't think I would have been able to retake it without holding Burgos (although I was running an offensive to attempt to do so when I took that screenshot). But yeah, I wasn't actually trying to take Burgos at any point, only to kick the troops out of it so that I could keep it surrounded more easily when I managed to do so and so that when I encountered them again elsewhere they'd be unsupplied
Funnily enough I first came up with the strategy playing the Kaiserreich version of the Spanish Civil War
I started in 1933. By 1935, I lost hald my tech team for no reason? Help.
If this is as Spain and the loss of a tech team coincides with the coup (I'm pretty sure in my case it happened on 17 June 1936 - ie the historically accurate date - but maybe it's possible-but-unlikely for it to happen earlier or later?) then it's because the missing tech team f*cked off to join the fascists, or that was my interpretation anyway. You'll also find that a good chunk of your military units sided with the fascists (it is a coup, after all)