Which accessibility standard for mobile app?

The W3C has created accessibility guidelines for websites. A is the least accessible, and puts the least constraints on the design, AAA is the most accessible, and puts the biggest constraints on the design. The W3C encourage to apply accessibility standards to non-website content also (Source).

I design a mobile app for iOS & Android and would like to pick an accessibility standard. My team and I are debating if AA or AAA are better. This will have a direct impact on design decisions, for example the choice of color. The WCAG contain minimum contrast ratios between text and background color.

On the one hand we want to be as accessible and future proof as possible, which speaks for AAA. On the other hand both Android and iOS come with accessibility features, which drastically improve accessibility independently of our color choice, which is why AA could be enough. For example, Android has a feature to increase text size.

Is it ok to say: AA is good enough, because users can increase text size with Android’s built-in tool, which makes lower contrast acceptable?

My question is about this specific example (color contrast) but also the broader approach: Can we be less strict about design choices, assuming that the OS’s accessibility features will improve accessibility for those who need it?