Hi Philipp, so, I checked in depth and revised your patch. Here's my take on it in a similar design of my last object.
I changed a lot of things and rewrote basically everything, so there might be something funny still and things may not match, but the basic stuff seem to be equivalent and the basic parameters like block size and delay seem to match.
anyway, this is also fully vanilla and the prototype is called [conv2~].
I am precomputing the FFT, so check it out, and also check the rest as I've changed much of your computations for something that's simpler I think.
cheers