I spent the whole day trying to dual-boot Ubuntu on my Mac, up to the point where I broke my GPT partition table and almost had to format everything (see [https://gist.github.com/vinicyusmacedo/4172cafc108bdfff7084017d1925b74f]). Then, I just found out installing Ubuntu on a Mac is really really easy. You need no gimmick at all (I'm not sure if I've just been lucky enough).
I won't cover anything in detail, but you need to, basically:
- Resize your APFS container (quite easy to do and non destructive) and keep some free space for Ubuntu
- Create an installation drive (follow the official guides)
- Install Ubuntu!