I'm currently designing a B2B app that will have plenty of buttons (on top app bar) that just can't be hidden. Is it possible to transfer some of the buttons from top app bar to bottom app bar, just like this?
Material Design is a design system for making design systems, paired with a set of guidelines. They are not rules. You've used many Material Design apps without realizing it, I'm sure.
Material's App Bar: Bottom appears to illustrates many of the layout concerns you have.
The Behavior section has recommendations on how to pair a bottom and top app bar together.