I have been in contact with Icons8 over the past month or two. They have now modified their simplified black SVG icons not to contain any fill attribute. Filemaker reads those simplified icons without crashing, they don't get filled and you are able to change color of the path in the inspector as expected.
Unfortunately Filemaker does not always scale the icons correctly from it's internal viewbox of 24x24 to that of the icons 50x50. I still have to figure that out.
PS: I'm not affiliated with Icons8 in any way. I just need icons the work without hassle. And they have a massive library, where you can download just what you need on the fly.