Correct. GTX 1080 Ti doesn't change anything. They also rely on the Nvidia web drivers.
The lowest AMD card that would work is the RX 560. But I think you can find really good deals on used Vega 56 cards on eBay now, if you want to get something more powerful. Before buying, check on the forum that the model you want to purchase is compatible. Some specific cards have problem. Quite a few XFX brand cards have problems with hackintoshes.
I think it's possible to use IGPU without pulling your GTX 1080 Ti, but it's something I've never tried to do personally.
If you need to drive so many monitors, you will need to use a dedicated video card. IGPU does not have that many outputs.