I think he's a terrible attacking coach.The advantage Tuchel brings is he can coach how to attack even if he is primarily a defence first coach. Southgate could never coach anything beyond hoofing the ball which worked with Rashford and Sterling but doesn’t with Saka and Foden.
He is good at using the players at his disposal to win a football match though, enough to get a team over the line in a Tournament (way better than Southgate).
Southgate was 100% on the right lines of how to win an International tournament but lacked that final in-game tweak to put the oppo on the back foot (I'm talking about vs a good side) or recognise or react to an oppo coach who's doing that to us.
It is in these moments where I see Tuchel way superior, he is usually the manager that the oppo is trying to react to.
When Southgate looked to play a more expansive style, when we were up again dross, he fell into exactly the same trap as Hodgson (with Neville) did which was to select players that played in attacking, high-pressing club sides and copy thair formations and then said go do what you did for your Clubs. They didn't have a clue how to actually coach it (Southgate better understood it than Hodgson/Neville), didn't understand pressing traps and triggers, couldn't implement attacking patterns and counter-press moves with runners from midfield etc...
The real issue is attacking football, and I mean properly coached patterns and rotations and well-designed presses and counter-presses are myths in International football. That's because the managers do not have the time with the squad to train it (it's why Club managers take two years before you see their own teams click when doing it - as per Pep, Artetta, Ange, Klopp & Poch to name a few).