In terms of quality, I believe this is fairly well done. The creator picked the phone messaging format and it has its merits - it aims for a sense of immediacy while also limiting information, trying to make it authentic.
But the phone format also has its limitations, which may become more pronounced over time. Personally, I feel that it already shows early on with scenes between MC and PC only being alluded to, so the direct relationship between PC and MC feels, on some levels, weakly established / maintained, while almost all visual and content focus is on other NPCs. Obviously, that is the content focus... but given that the game aims to embed this content in an (emotionally and sexually) stable, loving relationship, that feels like a definite weakness and something of an error. Something like "making videos together" or a memory section for the PC recalling some events with the MC, sans phone, might be worth considering.
The biggest design flaw, in my opinion, is the lack of choice or impact. The MC is established as entirely independent early on and the player has almost no direct impact or control on anything that occurs henceforth. If a choice is presented, it should be meaningful, at least short-term (e.g. telling her to not go out that night, to take things slower, not meet this person or that --> determining what content is unlocked while the overall direction of character development remains the same).
For that reason, I find it decidedly unsatisfying when choices are purely cosmetic with no impact whatsoever. Such choices are pointless and might as well just be automatic responses the player doesn't have to input. I've played too many low-effort games (that usually claimed high levels of interactivity) while offering purely cosmetic choices for this not to aggravate me on principle.
I hope the creator will keep improving as they keep gathering experience.