I'm using it with a Wacom One pen. My daughter used one with her stock SPen for a whille, but it may have killed its low-pressure sensitivity. Or it was just unfortunate timing. This is it: https://smile.amazon.com/dp/B0B5Y2D27C
(I just tried it in my SPen, and it didn't feel like it fit well. I was going to have to really push it in for the last 2-3mm, not at all like the stock nib. Sorry.)
I bought two of these. One for my S7 FE and one for my son's S6 Lite. I believe it should work for the S8 Ultra as well. They work great. Extremely smooth on the glass, a bit too smooth though, so I bought paperlike screen protectors and now they're awesome! I imagine I'll need to replace the screen protector in time, but it feels exactly like using a real pencil on paper.