Biggest mistake was leaving Rashford at home. That one I don't get. Your best team over the last year was Rashford-Jude-Saka behind Kane. I get he's had a poor season and Foden and Palmer have been great and you have to try them out, but still, at least leave yourself the option of a go-to lineup you know would work
Midfield and LB, I dunno. Aside from Mainoo and Wharton, who else is there that's been ignored? The latter two are both really young and weren't really an option until March, so, it's kinda understandable the manager may not fully trust them(he doesn't have a choice anymore though at this point. Surely) - though instisting with Trent was an abomination after the Serbia game made it patently clear that he can't start there. He hasn't got a clue. When he inverts for Liverpool it's usually after they've consolidated possession, so he doesn't receive the ball on the turn under pressure, and then the knows how to fit within their play. He's in there for his passing and yet was consistently ahead of the ball in these games, how does that make sense...
I digress. Point is midfield has indeed been a problem for England over the last year - that the options that should fix it are an 18 and a 20 year old with a season or less of top flight experience is proof of that, no matter how good you think they are - and I'm not sure Southgate actually could do much more about it going into the euros
LB, I see people mentioning a couple names so maybe that was an area where more could have been done. Dunno.
I think he should've just called and then played Rashford, would have gone a long way to fix a lot of the issues. That said I also think England is overrated quite a bit. Exceptional players but the best don't really fit particularly well together. A strong defence and relying on one of those attacking players to find a goal is probably your best option. Kinda like France. I don't think England have the players to play like Spain or Germany