I created a project with rec2020 HLG and imported my iPhone footage. I also want to include my footage from my drone but it's not in HDR. When I import it, it looks extremely dull. How can I use SDR clips in my HDR project / timeline in Final Cut Pro?
While you can try to use the HDR Tools Effect, depending on the input video it might not work very well - e.g. shadows might be a bit crushed, or the result otherwise too contrasty. You can try higher nits values, as one option.
I almost always get better results by simply using a Colour Board and raising shadows, midtones, and highlights by e.g. 5%, 35%, 60%. You need to tweak to taste and based on the source camera, but once you dial it in you can copy that Colour Board onto all clips from that camera.
Alas, to my knowledge, Final Cut Pro has no way to set any such corrections for all clips from a given camera; you have to manually apply them to every clip as you add it to your project's timeline.
As for the gamut shift you can use the inverse LUT by pointecc