I do feel for Southgate, he seems like he has a good heart, even if he isn't the brightest bulb in the chandelier. He deserves credit for the improved atmosphere within the team, and the players seem to like him.
Managing England is always going to be a tactical challenge because England produce so few 'positional' midfielders – it tends to be bravehearts who like to bomb around the field (Rice) or creative players who get end product and excel with positional freedom (Foden, Bellingham). It makes it harder to field an XI with real structural integrity, that can win the midfield battle against top sides and break down inferior teams happy to sit back in an organized defense.
The 3-4-2-1 was definitely an improvement, but even then it's tough to play both Bellingham and Foden. Rice and Mainoo are left to fight the midfield battle themselves (not ideal against Spain, of all nations!), and you lose some threat on the wings because the likes of Saka have to track back so much.
That said, you need to be able to bench Kane at some point. That can't be too much to ask when he's playing like garbage and seems to be carrying an injury. I would have loved to see 90 minutes of Watkins and Toney putting pressure on that rather pedestrian pair of Spanish center backs.