Darwin worked on the same problems a Mendel, but his own theories led him to come upwith very different conclusions. Neo-Darwinism sought to reconcile the two but at the cost of clarity. It is a bit like Einstein difficulties with quantum theories, but more like the old necessity of abandoning Ptolemy’s cosmology because it got so complicated that it could no longer be useful. IAC, Mendel, not Darwin, deserves the title of of “ Copernicus of biology.”
I think they were looking at very different but overlapping issues. Darwin looked at the big picture and saw what had happened and is happening. Mendel looked at the very small scale and observed what happened. Mendel had no understanding of evolution, even though Mendellian genetics came to explain why and how evolution works.