The Amsterdam start point for that express bus is Europaplein, so a visitor is likely still to have to get themselves there from central Amsterdam. I think that you can get from Rotterdam Centraal to Leiden Centraal by train in about half an hour, so it's probably not much less convenient than starting in Amsterdam. The OP would want to weigh that up against the hassle of staying in different hotels in different cities if it's caused only by visiting Keukenhof, plus the ease of flying to Rotterd