First of all what I'm about to say are my own opinions so take it with a grain of salt, in the end my point of view when it comes to game development and the game developers is that I very much prefer game devs to create the games that they wanted to create regardless of whether feedback was positive or negative. Just clearing this up right away.
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The pace is definitely too fast I feel. That really is the biggest underlying problem and flaw that I've seen. A nude beach plot device doesn't change this, it doesn't make the characters immediately comfortable with each other or at least it realistically shouldn't. With a seven days time limit the only thing I can see is a handjob being the most likely thing to happen and a blowjob at the least likely rarest.
The issue is mostly due to the idea that suggests that these characters, Yumi with Izzy and Nick, are almost complete unknowns to her. I would suggest revisiting this and get the comfort formalities sorted out before they all go to the nudist beach resort vacation. For example, you could have the couples go on a double date to "break the ice", Izzy and Yumi could go shopping for bikini's etc. This gives the characters time to get to know each other so that the audience (us) can understand and get a feel for the vibe they have with getting along with each other. How they interact and have a sense of cohesion with each other is somewhat important.
Another issue which remains to be seen in your works so far cause it's still so early I have no idea what your views of what Netorase is, but I'll say this much anyways just in case.
Most attempts at depicting Netorase in these games don't work. Mostly and primarily because the Dev has a minimal or very loose comprehension/ understanding of what Netorase is. Most devs seem to think Netorase is just couples swinging, and while that's kind of accurate, it's not the most direct definition of what Netorase is. Swinging/ Sharing is just what comes closest to what defines Netorase, but it's not a direct Japanese understanding of what Netorase is. Netorase is best depicted from the perspective of the siginificant other being stolen (in this case, that'd be Yumi I assume?), so the story in the NTS path would be taken over and depicted from Yumi's point of view and told to us audience as if it's a story that she is telling us, as Netorase is basically Netorare but it's from her perspective. Whether or not MC is a daft dense and clueless idiot or MC is clued in on the cheating and gets his rocks off on it is entirely up to the Devs creative choice on the matter.
If you were to include a Netori path, then the story would simply change to be from the perspective of Nick. Netorare would be from the perspective of MC. Netorase is generally best depicted from the perspective of the one being stolen (Yumi).
If you were going solely for a Swinging/ Sharing perspective on the story, I'd recommend just changing the tag from Netorase to Swinging I believe that the website finally fixed that issue and included them as tags for devs to put up.
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Now that I've reached the end of my critique, allow me to at least express that I believe you're doing a fairly decent job already and you don't necessarily have to change or do anything different. I like the art and the concept as is, I just feel that the pace is way too fast and that a nudist beach plot device to advance the plot doesn't necessarily do anything for me to see it being any different or better. Fact is, the characters supposedly know they're all going to this place called "Lust Haven Island", one would assume that they know or at least heard of this nudist beach resort before at one time or another, so they should know what they're getting in to. So it doesn't or at least it shouldn't present any fast shock value to set the pace of the story, especially since they all already agreed to go anyways without question and were looking forward to it. So I feel like them being "shocked" to find out it's a nudist beach resort feels like an unnecessary and forced plot hole to advance some concept to break the ice and be comfortable with each other unironically just falls flat.
Revamp the prologue, get them all to go on a double/ couples date, get them all somewhat comfortable with each other before going to the island, add in some extra scenes that put Yumi, Izzy and Nick in situations that expose them all to each other, this will easily break the ice a whole lot smoother.
Anyways, you've done well enough so far, and I still look forward to the next update. Keep up the good work!