I don't know if I am really getting this game. It doesn't seem like most NTR/NTS or whatever genre its supposed to be. Seems like its all about fun no malicious stuff. Compared to other stuff I have seen this is different and tame.
It's NTS which stands for Netorase. It is a Japanese word just like Netorare (NTR), but the difference is in the consent given by the partners. In Netorare, there is no consent from the partner. One partner goes behind the other's back and is cheating on them. The other partner usually finds out and is emotionally devastated by this.
In Netorase on the other hand, the cheating is usually viewed as sharing and is consensual. They also usually enjoy/get off on sharing their partner. It is hard though as IRL sharing involves a ton of trust as it can easily turn into cheating.
Both are examples of cuckolding, but they differ in whether or not one partner knows about it and consents to it.
This game is an example of Netorase, in which the MC and the girlfriend both love/trust each other, but also are into sharing. This game, from what the developer has told us, isn't going to be NTR at any point so don't expect Ayumi to go behind the MCs back and start sleeping with everyone, or starting a real relationship with Terrace/Terraza.
As for the dev diary, it's put behind the platinum paywall for a reason. It is a way for Torimiata to give back more to the people who support them more. It would be the same as leaking updates before Torimiata wants them to go public, and would be sabotaging their income for the time/effort they put into making this game. If you want to see those platinum images, subscribe to Torimiata's patreon.