How important is it to cite the earliest report of something?
For your readers, not important. Very often, later reports or review articles are better than the first report in terms of clarity and completeness. If you did not even need to read that first report when doing your research, then probably your readers do not need it either.
For the academic career game, mildly important. Depending on the field of research, citations may play a role in careers, and citing that first report may provide a (small) boost to its authors.
A pragmatic approach is to not waste too much time chasing citations, and wait for others to recommend citations that you may have omitted. Not everyone has to know the whole literature. And many people are not shy about asking colleagues to cite them (or others). To some extent, you can count on the readers of your article in draft/preprint/submitted form for pointing omissions.