This is without a doubt one of the best Stories i have ever had the pleasure of reading! (This includes Chapter 2&3 still on Pixiv as well)
[I'm also the one that left a comment there specifically because i unfortunately use a different username only on here, and i wrote it here in general because comment's on Pixiv can't be nearly as long as they are here]
This series combines literally almost anything i like, Dick growth (and later breast growth) Demons/Succubus and characters in these stories having the desire of wanting these things happen to them. (You can see that especially later in Chapter 2 when Cass injects the tentacles herself, i adored that!) All i can really say is that i beg you to keep everything going as is, because i already think it's perfect! Also in the beginning i didn't even realize it was made in Koikatu, your camerawork is really experienced and convincing as well, it looks much greater than average.
If i had to give any feedback, the only other things i can possibly think of that i want to see/improved in the future is perhaps a character that's really into what Cass is doing (Like she would consciously want the treatment Olivia got, on her own will without having to be convinced or something) and maybe a Hyper moment? a short one? And i personally suggest having their dicks not glow as often, to emphasize that they are real.
I saw chapter 6 go up thanks to f:Dick growth being one of my watched tags, and was surprised to find I hadn't read any of the prior chapters that also have it! This was REALLY good, and I look forward to reading the rest of the series!!!