I liked a good part of the Sapporo Hatsune Miku concert, the 2011 version. They made a lot of prgress since then regarding the adoption of lower tones and at the same time humanizing it. I listened to your last track and it's pretty good.
OTOH, I'm looking towards additive resynthesis, specifically Harmor and some libraries. Granted, these are pre-recorded. So it's much less flexible. Although some libraries also have aaahs and ooohs, which would be fine for me to start with since anyways I don't have words
Eg : (note that the picture of the singer is imprinted in the synth's harmonic output a few times here and there and shown in the synth's spectrum analyzer although they drop the audio for these passages)
I removed the direct link to the youtube video demonstrating the Harmor synth with vocals, hence removed a very large picture display which seemed incongruous as I wanted to only link to that vocal technology and not taking the whole place. Instead of the link, a search in youtube using 'HARMOR Victoriya Vocals Resynthesized' will yield the demo.