500 fps takes special gear. are u sure it wasn’t shot at 1/500 second?
If it’s really 500 fps then time remapping is easy because 1 second in real time will have 500 frames so you should have no frame blending problems or extra frames
just set keyframes where you want to speed up and slow down then drag them around by selecting all keyframes right of the point you want to change and work from left to right.
did that make sense? after you get the timing the way you want you may want to ease in and out.