April vs June: I would say ….. it depends. My husband and I had jobs in education, and so, except for one trip at Christmas, we always went to Disney in the summer. And, then, we retired and went to Diseny this past April. We were there two weeks, and Easter was smack dab in the middle of our holiday.
Most definitely, April had the larger crowd size. And, on Easter, Magic Kingdom was packed. I mean, we could hardly move packed.
However, the weather was gorgeous. It was shorts wearing weather, but there were mornings I took a light jacket.
If it were just my husband and myself, like it was this past April, I would say-go in April. We didn't make any fastpass reservations until we arrived, and we got fastpasses for everything we wanted to ride, except FoP. And, except for Easter day, the crowds were manageable. We did a few sit down dinners, which we got reservations for the day before. But, again, there were only two of us. AND, we went with the flow. We didn't worry about cramming everything into one day. And, if we thought a line was too long (except for FoP) we skipped the ride. We did Disney in a most relaxing way.
Now, if we had been going with our kids and grandkids, although April still would call out to me, I realize I would have to do more planning. Those fastpasses and restaurant reservations would have to be made ahead of time. With a larger group of people, there is less ability to deviate from "the plan." And, I thought it was a bit chilly for swimming.
Hope this helps.