If I’m correct we’re talking about an imac 9,1.
According to this post:
‘In life mode everything seems to work.’ (but issue dd vs etcher)
That was only 4 month ago and they boot, as you experienced with other distros, normally without issues. I’m writing this on a 2009 mbp, different hardware/no MacOS but same 64bit efi, boots manjaro isos fine without refit.
- try different stick (seriously!)
- check the way you burn (suse imagewriter works)
- use the LTS iso with 5.10LTS kernel