Not technically a fountain pen, but the Karetake No 13 brush pen has a refillable converter and takes fountain pen inks.

This is only my second ink, but I am amazed by it so far! It is a vastly wetter ink than the Karetake black cartridge, and it bleeds through quite a bit cheaper paper, at least with the bath that a brush pen provides. My Leuchtturm1917 is robust enough for it, though. So far, I’ve been pairing it with iroshizuku murasaki-shikibu (Beautyberry) in a Lamy Safari. It is a lovely contrast in my opinion.