Hello again Giannis,
Nice to see that you are getting underway with EPD creation. I have some notes to the chosen processes.
1. In general you would go with a set distance for the disposal. Think of it like a scenario in classical LCA. Perhaps you would model 150 km to disposal, for a declared unit of 1 ton, equalling 150 tkm of transport with a representative transport process. This would be your C2. A general process might work, but is not as transparent.
2. I agree with your choices for C3 and C4.
3. Module D is avoided material or energy production, so the process of recycling does not fit into the process here. If the concrete is recycled, then the disposal process in C3 is the one you currently have chosen for D, without a C4 process. If the concrete is recycled into e.g. road gravel, then for module D you include the production of -1 tons of road gravel. This does not need to be produced now, as the concrete has been recycled to take its place. If your material is landfilled, there is nothing in module D.
I hope this helps, and good further luck