Thanks again for your help!!!
I tried using the scaling. But it is really hard to control on multiple devices and ratios etc.
But I was able to get a reliable result (a little bit brute-forcing it) by resizing my input UI on every tick the Input Mode is active. This is due to the fact that especially Android is "readjusting" the app on multiple occasions (when opening the keyboard, sometimes when the input field is focused, etc.)
I feel ok doing so because everything else in this mode is not really performance heavy - it gave me the most reliable results.
I will post a screenshot from my code, but it won't be very representative. It just shows my responsive scaling:
But here is how it looks on IOS
I will most likely share the whole file once the app is finished and maybe do quick tutorials on certain aspects of the project if people are interested in it.
Good night everybody ;)