He is immensely risk averse, which to be fair has served him well. Teams go far in tournaments by not losing and he is clearly a fan of playing the percentages at the international level.
Tactically I think he is very limited, though arguably that is the nature of international football vs club football with the amount of time a coach has with his players. He clearly is defensively minded.
I do find it very frustrating that we can have some incredibly talented players like Bellingham, Grealish and Foden, an embarassment of riches most countries would want, that appear to consistently be stunted by the style of play.
I don't blame him for being loyal to the players he has been loyal to, they have served him well and are his men. He's clearly also a man who understands that the best on field 11 players or best 24 for a squad don't necessarily equate to most success. He's picked "good tourists" and dumped those who have stepped outside of that - Ben White for example. Talent isn't everything to him, and it has made the England team feel much more likeable than it has appeared in the past 2 decades or so.
Its been more successful than any approach before or since (save one tournament in 1966 or maybe Italia 90). In 2018 he had the benefit of low expectations. In 2020(1) we reached the final and got beat by a similarly defensively minded side, 2022 we only just got beat by arguably the best squad in the world. We also won a penalty shoot out in 2018 - so he's doing something right.
Its difficult to say what happens next really. England are one of the best in Europe and the World, certainly good enough to win international tournaments. Southgate's style seems to get us to these big games, but I'm not convinced it allows us to win them.