Losslessly Changing Video Framerate

I’ve been taking a few timelapse videos recently and as an amateur I’ve just been using the default phone camera apps. Google Camera defaults to “Auto” for the speedup, which keeps the resulting video between 15 and 30s no matter how long you record for (with minimum of 5x speed). It also offers explicit 5, 10, 30 and 120x speeds. The iPhone forgoes explicit options and only offers an auto mode…