I think the solution is to pack the middle of the pitch, much like Leicester did yesterday.
Sigurdsson playing so high up isn't doing much for us. It limits our passing options through the middle during build-ups and leaves our midfield exposed without the ball because we don't win it back in our opponents' third very often. Fortunately, it should be an easy fix. If Silva doesn't want to drop Sigurdsson, he can position him further back so he can get on the ball more and help protect our defense. Or if we want even more defensive sturdiness, especially against quality opposition, he can bring Davies into the team for Sigurdsson.
We have solid evidence that the league has worked out how to play against the 4-2-4. We managed to get one nice result with the 5-2-3 but that still leaves us exposed in the middle and makes it hard for us to control the game (Brighton were able to take advantage of this). The next step is to try a proper 4-3-3, where all three midfielders play deeper with and without the ball. If that means buying a new midfielder this January then so be it (this continues to be the position that requires the most improvement, not CF or RB).
I've seen plenty of criticism of our front three but I'll back Richarlison-DCL-Walcott/Bernard/Lookman to score enough to get us good results given more of a foothold in the middle of the park and more territorial control over matches.