They really just teased us a big vampire meeting and then completely ignored it, huh?
Also, calling her Sacchan is fine, but Saki is problematic?
It kind of is.
Honorifics in Japan are a sign of respect (and also distance) to people you know.
chan
in
Sacchan
is a "we are friends close enough we can shorten our names, but not short enough we are more than that"
Calling someone by their first name means familiarity. Either you grew up together, or something else happened that made you be more than just close.
When you give permission to someone else to call you by your first name without honorifics, you are telling that person you feel as close as family (partners and lovers are included under that term family too). It is ambiguous in terms of boundaries, but you do intend to make them as present in your life as possible.
That's the reason Kou said he isn't "ready" for that, as in, he doesn't know if he feels as close to her to be considered her family.