Yes, with or without Ten Hag massive investment is needed.
If someone could give me a viable option for who Ineos might actually for, then I could be interested, until then I see no issue in just sticking with what we've got.
I know his stock has fallen recently, but I still think De Zerbi would be a better option. Ten Hag has said he can't play at United like he played at Ajax, which to me basically invalidates the logic for which he was hired in the first place (based on the way he had Ajax playing).
I don't think De Zerbi would compromise on his preferred way of playing and I think he would know what kinds of players he needs, which is the first thing that United needs right now imo. Regardless of the changes above the manager, there has to be a clear vision toward which things can move. Ten Hag signed players he had before, which suggests a plan to replicate what he's done before, but then he completely changes style and says he can't play that way. So where exactly are you headed?
Even without signings, De Zerbi would come in and add structure and have a clear plan, drill the team in training to play a certain way, improve players who are already there. And then I think he would be able to say yay or nay to proposed signings on the basis of whether they have the correct attributes for the system he wants.