Many say it is a Spin-Off, others say it is the prequel to what happens in the first three original "Planet of the Apes" movies, which place it in a timeline. Anyway, like you, I didn't expect to see more movies and more of this franchise, which did an excellent job with the Caesar trilogy and with the success of this fourth film I look forward to what they have in store for the future.
Excellent review.
I certainly think that as a standalone it works well, it doesn't add much to the stories already told, but it's fun to watch.