I used to be a plan/reserve everyday kind of traveler.
I’ve evolved to just reserving for destinations that are difficult, like anything within three hours of population centers in California, Yellowstone in July & early August, etc. For example, got reservations for Yellowstone around the Eclipse 18 months out. Or 6 months out if I want to camp in Yosemite valley (which is well worth the effort)
That said, I was quite successful with an impromptu week early August visit to Yellowstone in 2015 without reservations. Only drawback was spending extra time focused on finding sites instead of viewing the park. But then I enjoy primitive campgrounds, which helps considerably with finding places.