Not really if you accidentally commit to one girl it removes other ones chains so no it also doesn't have sandbox in that sense either...
No, please take notice of "Harem" tag.
Like it has decisions, and options but is still very very much linear over all...
No.
Sand box would be me forgeting the main quest and chasing down a bunch of things, only moving forward once a blue moon to get further into side things. This is like "you got side stuff, but heavily relies on the main quest line in order to do any of them."
There are big sandboxes and small sandboxes, this one is on the smaller side; You are right to say it heavily relies on the main storyline, but there is not just a single way to make a sandbox.
There are bigger sandboxes where the side stuff you do have no impact on the storyline, I preferred to have a smaller one where the choices you make in those side events do impact the storyline.
There is also to say that not everyone likes to get "lost".
This game was originally born just as an experiment to see if I could fix some of the biggest problems I noticed from sandbox games (Getting lost), then it turned into an actual game since people liked it.
Like to me sandbox would be like skrim where I never ever beat the main story line and got hundreds of hours in it anyways lol.
This is a project made by just one person (me), there is no way I would be able to make hundreds of hours of side content.
It would not work, either. This is a pure visual novel, not Skyrim. It has no gameplay elements, just reading.
When it comes to visual novels, the big "AAA" ones, they are typically 30-60 hours long.
Its where I can freely do what you want, this is like hey you can do 2-3 things between main story line progression! I mean I like it but I never noticed the tag till I searched for sandbox games and I just laughed when I saw some one exagerated hard core to add this to it.
A good example is with tags male protag and sandbox its next to this one is Wife Trainer where you can do any main girl line you want in any order, lessons of love same thing, Public Sex Life, HS Tutor and Trials in Tainted Space, but then you got this game in the mix for some reason and it just doesn't live up to the rest of the ones using the tag that show up just on page 1 of those tag filters... Its like using NTR for a FFM threesome, its just kind cheating the tag system.
I did not play any of those games, so I cannot say anything about that.
However, sandbox is just a design style and it can have various ranges of freedom, this game qualifies as one.