I think squad quality isn't the issue at United. Ten Hag, Antony, Casemiro, Varane, Sancho, Martinez, van de Beek were very good signings at the point of their announcements, IMO, and many players with a longer history such as Rashford, Garnacho, Shaw, Bruno or Eriksen proved that they are capable of playing football at a very high standard. Yet with a few exceptions, all of them underwhelm. There has to be a root cause for why so few players and managers fulfill their potential and until you've solved that problem, it won't really matter who you bring in. A player who currently plsys well for Bayern will most likely eventually be a shadow of his former self after joining United.
My guess is a clear lack of ambition and expectation from within. And primarily from managers who for some reason don't believe that their work is to actually coach every player and maximize every single one of them, when a manager has low standards for himself how can player have high standard? Also, and I'm guilty of it, everyone becomes a hero after a couple of good games, look at Garnacho or how things are turning for Diallo.
Manchester United is a low standard Football club.