I know where you are coming from. I use Korg Gadget quite a lot for the easy preset library. But Audulus is the app I go to when I know what I want to do and don’t want to be bothered to learn how to do it in one of the many fixed architecture synth engines. Also, you can design your own synth much more so than any other app. I mean to the level of, “I don’t like how that wavefolder works, I’ll make it asymmetrical and saturate it a little.”
I should also add that I like to build on the Mac version and play on the iPad, especially for modular.