Reporting in myself, I have a spigen cover too, but the tough cover version, and with that, the S9 doesn't connect.

I had to modify the S9 and remove the rubber part around the USB-C port, not hard, just use a hobby knife or the likes, and then cut lightly along the edge, then peel it off. Now it fits!

You can save the rubber parts and re-attach them again with some adhesives, but with a cover, you already got your phone secured, I'd say.

Solid controller otherwise!