Hi Maffers80,
GPU acceleration is done with Vulkan, which should work with AMD cards if the driver supports it. Here's some official info about Vulkan driver support for AMD
The models used in the app are simply a subset of the current ones. Largest bundled model is large-v1 as those were the ones that created the best outputs, and I don't want to increase even more the size of the app, the slider goes tiny→large-v1.
If you want to use a different model, you can do it by simply overwriting the model file itself (keep the same names though for the program to work). For example you can overwrite large-v1 with large-v2 and when the slider is at the most accurate one it will use large-v2 (it will load a model called large-v1 but the contents will be the large-v2 model).
That way you should be able to run any similar model that you want.