Seems to me that UMich will be fine for what you want. Do some merit hunting in the US if you want to, focusing on cohort type scholarships - if you really have what it takes to put in a credible application to Oxford then you ought to be a strong candidate for merit in the US. In any case I don’t see PPE or Law in the UK as a particularly good route into investment banking, PPE is more classically thought of as a route into politics or journalism, while law is a vocational degree.
I’ll just add that we went through some of the same decisionmaking process, concluding that it wasn’t worth spending $80K per year on college but Oxford was an option that was not meaningfully more expensive then our instate flagships in CA. After being turned down by Oxford, S took the instate flagship route, he is very happy and has a 4.0, prestigious internships, etc. It has all worked out very well.