We must improve both attack and defence. Most of the conceded goals and lost matches came when we played other CB formations than Harry-Lindelof. When they played we have an average of 2.15 points per game, compared to 2.25 for City. When one of them is missing, we have an average of 1.5 points per game. We need one more reliable CB, but not necessarily a top CB if that will stop us from improving other areas. We had many draws with clean sheets and need to improve our attack. Sancho would be perfect.
All together I would prioritise a world class midfielder. Think that would improve both attack and defence.