My first impression was that this was yet another troll post, but after rereading it, I've come to the conclusion it is an opinion post with some snarky and judgy elements. Since I'm feeling magnanimous today, I'll try to refrain from an equally judgy/snarky reply and try to address your issues, at least on some level.
With regard to the inability to have all the island girls on the island at once and the need to rotate island girls on and off the island, there are logistical in-game QOL reasons for doing that, as well as a significant amount of content associated with particular girls leaving and returning; beyond that, I'll leave it to
Darkhound to address the particulars of that in more detail, if he is so inclined, since after all it is his "design choice".
With regard to "grindiness", honestly this game is as grindy (or not) as the player chooses to make it, based on their playing style and choices. There is an unbelievable amount of content in this game; some of it with prerequisite dependencies, but much of it without any prerequisites. Players who approach the game with a "linear completionist" playing style are probably going to find the game pretty grindy based on their playing style. On the other hand, players who take a more "Zen" approach and basically "let the game come to them" while using the information found in the
Companion Guide (see OP for link) to help them complete certain in-game achievements along the way will be much less likely to find this game grindy at all, and will get hours of entertainment from playing it.
And finally, with regard to the game being "very temperamental", I'm inclined to think that was basically a bullshit comment included in your post for rhetorical purposes. That's just not this game's reputation. Its backward compatibility with saves from previous versions going back to versions from the early days of the game is among the very best on this site. It has a quasi-official, Dev-supported mod
(Companion Mod, see link in the OP) that is mature, stable, and compatible/well integrated with the game with very few revisions since the early days of the game. The beta testing process for game updates is excellent, and any identified bugs from each update release are quickly addressed and patched in a subsequent bug-fix version that is typically released within two weeks or so of the initial update release.
While this game has something in it for almost everyone, not everyone is going to find it appealing, and that's okay; there are lots of games out there, to cater to pretty much every taste. So, try Holiday Island again... or not. It's all good either way.
