Well I had the same case but a different camera. I recorded some HLG3 files with my exposure metering at 0 using multi metering mode on my A7III. When I imported my files into my timeline in fcpx I used the HDR Standard tool to convert it from 2020 to rec 709. I think it is not the best method but it kinda worked. I used Filmconvert for my grading my part but had to put my exposure down in post by 2.5 stops.
Here are my results: