The problem isn't sandbox. Most of the biggest and best harem porn games are sandbox ( harem hotel, the headmaster, dark magic, treasure of nadia, corruption, sisterly lust, lessons in love...etc). There are some amazing linear games but since linear content can be quickly consumed and you have to wait months for new content, it's dissapointing for a lot of people. Like for example even no updates for months, you can return harem hotel and fap to it time to time since there are hundreds of repeatable sex scenes in the game. The problem with this game is that there is very little repeatable sex scenes. You can't have a sandbox games with so tiny amount of repeatable sex scenes. There is no point to play this game after you unlock the scenes since there is nearly zero repeatable sex content. That's it. Nearly all the sex scenes in this game are non-repeatable scenes. Which is kinda bullshit if you ask me. Still I love the game despite that. I hope we will get more repeatable scenes in the future. Because as it's, I cannot even consider this game as sandbox because how little repetable sex content are
Sisterly Lust was never a sandbox, it's a VN. Nadia is a de facto RPGM game. Out of your list, Harem Hotel stands, the rest being called the biggest or the rest, highly debatable.
Most importantly though, calling VN-s linear games is simply not true. That's the biggest urban myth that some of the people around here is hell bent to believe, because there's a massive misconception about what linear means. There's an important difference between linear playstyle, and linear storytelling. Sandbox allows you to choose the order of the events, but that's it. While a branching storytelling allows you to go on different routes in said events. If anything, the majority if sandbox games are linear by storytelling, because while you get the impression of freedom because you can choose the order, you often don't get to choose the outcome or branches. While in VNs, you often don't get to choose the order, but because of the different branches you get to choose the routes, and ultimately, the vareity in the events will make you feel less linear than just being able to choose in what order you do them. A game that isn't truly linear in any aspect is one that has both of these attributes, which is by the way super rare. Also, sandbox style of very often used by developers to create big ass maps and clickfests, hiding the lack of content behind them, stretching the playtime by having you finding them, instead of actually having a big amount of content in each update or the game as a whole.
