Seryn/Ceryn is lovely! I donβt have a preferred spelling, but I do associate both with different genders. I can see Seryn working on any gender, but Ceryn feels a bit more feminine to me.
I like the look of Ceryn better, but I also think people would mispronounce it as Karen, since Ceryn looks a lot like Carys (which is pronounced with a hard C).
I think Seryn is pretty nice, not a massive fan of Ceryn just cause it kinda looks like itβs supposed to pronounced similar to [name_f]Karen[/name_f]