I'd recommend Da Vinci Resolve for making timelapses. It performs really well and let's you scrub through before rendering anything which lets you clip just the part that you need. Plus you get the benefit of high export quality which can be fiddly with ffmpeg.
I would argue that ffmpeg has the hardest to understand interface in the video editing space (unless you can find a command someone else came up with that does exactly what you want)
there are a bunch of flags to get exactly right in order to get it to give you a high quality image out. there are wrappers to do this more easily for you, ffmpeg is a low level tool.
