Ramsdale would be a much better GK to switch to after the Euros, as he's actually a technical goalkeeper, who's good with his feet and at passing. If England want to play possession football and dominate on international level, he's the one to start bedding in, if Pickford is binned. Pope is similar to De Gea, that's why he'll eventually be replaced at Newcastle.
If you want to start phasing out Maguire, it would be infinitely better to have Stones at RCB and have Colwill or Branthwaite at LCB, instead of Tomori, who I feel like is worse than every other name I just mentioned, anyway.
Not sure about Trent. I'd rather have him at right back. If the coach can make it work, he can be a priceless asset for England. The other options are great as well, so it's not like Southgate can get it totally wrong. Walker, Trippier, etc. are all quality options.
Foden is not ideal on the left. Rashford, Grealish, Sterling, Gordon all better options there. If you want to find a place for Foden no matter what, it should be at #10 with Bellingham dropping back to #8, but then Mainoo gets left out.
My team for the Euros would be: Pickford - Walker Maguire Stones Shaw - Rice Mainoo - Rashford Bellingham Saka - Kane
After the tournament ends, you can start phasing out the older players if there's a great player you can replace them with. Colwill and Branthwaite are both really good and left footed as well, so it would make sense to have one of them replace Maguire eventually.