This isn't a criticism or anything, I like pretty much everything you've said here, but I always advice caution when it comes to hookups and side girls. Because if you mean side girls as in the Mc bangs them and they are only used for early sex scenes or something and then they disappear and don't have any kind of role in a harem ending and get left behind, then I (and some others too) would just as soon not have them show up at all and have a scene or more time devoted to a main girl (especially if the side girl is like a hooker or something, which just comes across as odd when the Mc has other girls at his call, going to a whore seems strange). Even a simple date with a main girl is better, in my mind, than a quick out of place sex scene with a random side girl who amounts to nothing.
But if by side girls you mean that they just don't get as much screen time as main girls, but still are part of the harem and are loyal to the Mc, and have at least an appearance in/towards the ending then that's fine. Not every girls needs to have the same amount of content and time devoted to them. I just hate when girls get left behind.
Side girls can be a double edge sword, because we, as players, may not know who is and is not a side girl or main girl. Like, we can assume that family is all main girls, but any other girls? We may not know, and so I've see it before, where a dev makes a girl they plan to have a side girl (and even the kind with one scene and then she disappears) but the players really like her and want more, so the dev upgrades her to a main girl (or at least not dismiss her completely from the story) or just loses people who wonder why their favorite girl "got cut from the story." This can especially happen if the side girl is very attractive and/or has a great early scene and personality. Players often connect with a girl the Mc has such moments with early on because, well, she's already gotten scenes, the players and the Mc in the game have already been with her and so it is easier to develop an attachment to her and grow her as a character in other ways since that obstacle is out of the way, so to speak. For a concrete example, look at
Game of Inches, a girl who was supposed to be a summer fling side girl has become on of the main girls in that game now because of how well received she was and how much the dev liked making scenes for her.
Anyway, rambling aside, those are just my thoughts I just wanted to share about side girls. It's not a big deal, just something to think about. Fixing to play the game and am excited to see where everything goes so, again, this is just my ramblings since idk how anything is implemented yet, let alone the plans, just talking things out.