You can go into investment banking with any degree, including bioengineering. Also research has found within 10 years both STEM and non-STEM graduates have equivalent salary and career outcomes. Equally starting salaries don't tell you anything about long term prospects or salary outcomes.
You can live a comfortable middle class life as an engineer, and you aren't going to have any greater life satisfaction with a higher salary than an experience engineer will earn (as research has shown beyond ~£50k at the time, probably closer to £60k now, self reported life satisfaction plateaus as in line with Maslow's hierarchy of needs, your basic needs would have been satisified at that time with that income and further income is not going to fulfill higher psychological needs).
This is even before considering that unlike engineers, investment bankers are going to be expected to be pulling 100+ hour weeks of face time etc (where you aren't being paid overtime or anything like that) and having no personal life, while living in one of the most expensive cities on the planet. Whereas an engineer can live in a much cheaper nice small city somewhere up north or in the midlands and have their salary go a lot further.
The fact you seem to have done zero research, have no idea how to gauge salaries and progression relative to actual life expenditures, have set an arbitrary goal of "being a millionaire" which has no relation to any kind of life satisfaction, and don't seem to understand what the requirements are for the roles you want to do or even the degrees you would be considering, are all frankly bigger barriers to becoming wealthy or achieving anything really.
This reads as the thoughts of someone who has lived an extremely sheltered life and has very little awareness of the value of money or what else is important in life. There are also a lot of other factors that will change across the lifespan which will shift the relative values of what income you need to be financially secure, but in any case it is far less than earning millions a year.
Also, it's certainly possible for a middle class engineer to save up over their entire life time the value of a million or more (in fact I think it would be surprising if they didn't?) through property ownership, pensions, company benefits including stock schemes, etc. Which would make one a "millionaire".