We planned our last trip to the Netherlands abd Belgium specifically to see the Bloemencorso, but we didn't go to the Gardens until the Monday after. It is a zoo on the Saturday of the parade, Sunday and probably for a few days before that weekend. Monday was not crowded at all.
I think the parade is well worth the time, especially if you watch from someplace a bit away from the Keukenhof entrance in Lisse. Here's a link to the parade route and timing. Scroll down to last year's Folder Bloemencorso. It's a printable pdf. They are always later than we planners would like with the current year information, but 2019 gives a good idea of the route.
We stayed in Leiden, had a car and went early enough to park near our chosen watching location. That was near a sports complex where there's a sharp turn on the green of the map between #2 and #3. We saw the floats and marching bands come around that turn.
There were so few people at that location that we could step out into the narrow street to take pictures. One of my favorite things is the fragrance of the floats as they pass by. The ones with the hyacinths are amazing.
After the parade passed by us, we got in our car and headed for the flower fields. We got to see some of them, but many were blocked off. An unexpected sight that sunny, but very cold day was an auto group driving parade-style on one of the roads. Why was this unique? It was 50's era American cars, mostly Studebakers and Cadillacs.
Whether you make it to Keukenhof or the flower fields (also accessible by small boat tours from inside Keukenhof) or the parade, be aware that if it's been a cold spring, there may not be many tulips blooming. You should have better luck than we did with your late April arrival.
The Gardens have massive greenhouses with blooming tulips and other flowers. Those were as interesting to us as the outdoor plantings. Here's a link to Keukenhof for more info.
We flew out of Amsterdam on the very day in 2013 that Queen's Day became King's Day. That was a really big event. Amsterdam was our last stop on our trip. We spent a week there, so we got to see lots of pre-celebration celebrating during that week. I don't know how King's day is celebrated elsewhere.
I couldn't resist looking up Zeelandic Flanders and when I did, I realized that we probably drove right through Zeeland from Bruges to Rotterdam. Knowing our penchant for backroads, I'm sure we were on N253, N61, N62, N254 and N57. We loved all the water and long bridges.