I like Cillian and Killian is ok as the anglicised version, but the Kylian spelling is one I just dislike.
It reads like it should be pronounced 'Kylie Ann'; even knowing the intended pronunciation, my brain still wants to start with 'Ky' rather than 'Ki'. And that sounds like a Kylie Minogue fan was disappointed at having a boy and wanted to shoehorn in a Kylie reference anyway.
Cillian or Killian reads much better, lacks the 'trying to be younique' cringe factor, and wouldn't date like Kylian may well do.