My feedback:
Idea - The premise is simple, but very different than every other typical VN setting: Two girls life by the coast dream to be a lifeguard, instead of the typical "schoolgirl in the urban city" which has been done to death. Even among the FMG genre, you avoid the overdone serum trope, and instead use a lost jewelry, injecting a dose of mystery into your story. I compliment your creativity to explore an unfamiliar territory (for me at least) and very much intrigue what comes next.
Visual - Despite utilizing AI generated images, they're still beautifully made and has little to no flaw. Additionally, you take an extra step in animating the scene, (however simple as it is) making them come alive. Well done. My only criticism is that the character image for Gram seems very off as from what I understand is Natsumi's grandmother - she looks too young; need more sag and wrinkle.
Writing - In the short demo, I immediately knows the character very quickly: Natsumi, Tangerine, Lovel, Grams, even the despicable Nick. They're nicely written, no apparent grammar or spelling error and lively. And I appreciate the little text animation you put in.
Conclusion - It's difficult not to draw comparison to another the established FMG VN like the StrongGirls series in term of style and quality, however I can see a huge potential and I acknowledge how much love and care you put into this. Despite of being in a very niche genre (Muscle Girl in Visual Novel using AI generated assets), I can happily say that there is another addition in the works by a very capable hand. Keep it up
Before I end, I have a questions: Do you intend to make it a full life sim, linear story or hybrid like the StrongGirls VN?