Sandbox design for H-games is a developer trap like 90% of the time. Even the best sandboxes inevitably devolve into something more kinetic when the dev has to run the football, because most devs lose track of what motivates players. Sandbox might theoretically be the ideal for making "a forever game" that you can make endless updates forever while getting patreon money, but the graveyard of abandoned sandbox games on this site should warn people away from trying that without a really good idea of what they are doing.
sandbox games are best for harems (or reverse harems) where you can present several options for the player to interact with based on their personal preference. The ideal sandbox is basically its own RPG system, with freedom to engage with a game world as you see fit (i think masters of raana is the best example of this, but i don't really enjoy the kinks that game heavily focuses on, so it's not worth playing). Read that again: the literal best example I could think of is not worth playing because it's too heavily focused on kinks that don't interest me. Think really hard before spending a ton of energy on scoping out a sandbox.
So like, if your goal is to have a wide selection of scenes, lots of different body types, lots of different kinks (even mutually exclusive kinks like sharing vs hard NTR, or wholesome romance vs aggro male dom), then a sandbox is a good idea. even then, the second you start dedicating more time to one track rather than another, you're fucking up the whole reason to use sandbox design
if your goal is to develop an extensive relationship with one character, or a smaller cast (like less than 3) than you should strongly consider NOT using a sandbox. It's a trade off. If you make a sandbox, you are letting go of the direct chain of actions a player might take. If you want players to experience a steady paced series of encounters (say gradual corruption, gradual seduction, or befriending an initially hostile character) just make a good kinetic novel instead. there's no reason to completely surrender control over to the player to pace themselves, just for the player to get annoyed because evreything has stupid checks like "mornings only do this, afternoons only do this, evenings talk to this one, oh no you need to advance several in game days to get the $$$ required." that's just a kinetic novel with extra timewasting steps!
sandbox should be, here's where these 5+ different characters hang out, which one do you want to see first? that's it really. if your story is long and heavily gatekept by conditions, you might be better off just writing the story out in advance and structuring it FOR the player, rather than hoping that the player decides to play it the way you imagined