I think everything Kaya did from the beginning of his solo career to Gothic (2013) was good. After that, dunno, bit too poppy with some exceptions.
Kaya's vocals have always been pretty solid. "Kugutsu" is probably my favourite song by Kaya and his vocals are great in it. I also love his voice in "Sink", a b-side on the "Awilda" single. I'm pretty sure that song was never on any of his albums, which is a shame as it's one of my favourites.
I do also love when Kaya does something new with his voice, like "Sorcière" or "Sugar Rose". With Schwarz Stein Kaya had to sing more or less in the same style constantly, so it's nice that he does other stuff during his solo.
New Vogue Children has such an unique athmosphere to it and Kaya's voice is such a perfect fit with the music. However, his vocals are just buried under billion different effects, which works on the album perfectly, but sadly makes it a bit hard to judge them. With the newer SS releases Kaya's vocals are a bit clearer, but that's one of the reasons why it doesn't sound like the old SS anymore.
I have to admit, though, I've recently warmed up a bit to the later SS releases. Immortal Verses especially.
Also, I got the Kaya calendar for 2021. The tradition continues!