Think of material design as the basic tools that you need to get started. It has expanded and developed significantly in the last four years and will likely continue to do so. You just might be ahead of the curve.
Material design was intended for the android platform and Apple's Human Interface Guide was intended for the ios, mac,... Platforms. With the introduction of cross-platform front end frameworks, React Native, many people do not create platform specific user interfaces.
Your theme should represent your brand. https://material.io/design/material-theming/overview.html#material-theming
Create a color system that is consistent with the brand. Material design color system did not work for me. I was looking minimal color in my app as it is mostly data so I have one primary color only.
I used the standard ios and android fonts except where branding was important.
I recommend sticking with the navigation components as much as possible.