Mad King
Let me start off by thanking you for the wonderfull game and your invitation to have a constructive discussion about your sandbox choice. I personally feel that more "player choice" and "decision impact" are very good goals to aim for and i think your game would profit from them immensely. However i also share the sentiment of many others who got burned (on other games) by sandbox features.
Not because the feature in itself is bad but because it often is implemented lazily and developed in a way that derails the story.
For me there are 4 lessons learned / fears from that i would like to share with you so you can reflect or dismiss them if you already found solutions we have yet to see.
1.) Spreading out development ressources
As the title says, so far you were 100% able to develop your game and the main story with as much details and length as you want it to be. Having a sandbox system usually results in a "main storyline" which will be progressed in every x-monthly update and a bunch of sidestories often locked behind sandbox features. I.e. the scenes with the priestess locked behind level 2 temple as a random example. This leads to half of the development ressources (your updates usually have only 3-4 scenes) to be split both towards the mainstory and justifying the sandbox feature. This often ends up feeling like the sandbox is just tacked on and stretches the mainstory thin.
2.)The illusion of choice
Like i initially said more choices empower the player to have an impact on the story. While a sandbox might have this vision for a player who picks up Vis 1.9 in a couple of years and who can readily choose to upgrade the festivals first or the temple and so discover all your content at his own pace the reality is that this is for us, your existing playerbase, an illusion. Limited by your workhours your future changelogs will most likely look similar to something like this: "Mainstory progresses by talking to Morgana, added a sidestory at the temple with the priestess (temple lvl 2) and added another sidestory for the harvest festival for this update." This means every current player will play through the newly available content. Without choice. In short despite the sandbox every update will be linear since its size is limited by its content and ppl generally unlock all scenes before the next update hits.
3.)Its either or, delayed progression
Having the sandbox taking up development time will mean that many of our favourite girls will probably get much less screentime overall. Afterall what kind of sense would it make to limit morganas or moms story by upgrading our farms? This leads to a dissapointing predictability of your future updates for your audience. If you know this update is about morgana, there will not be much mom or lil sis content and vice versa. So far all your updates were big black boxes for us "where will he take us with x character". The sandbox quickly gets the short end as either a wasted opportunity (because it does little for our main girls) or if it does it feels tacked on and just a chore.
4.)Chores and Featurecreep
Many other games use sandbox features to stretch their content. An update might just have 30 min texts and scenes but by making you grind or unlocking features/currencies the playtime gets stretched to 1.5h because you need to cycle through 5 weeks to get all the tuesdays or currencies covered. This is very very boring and sadly ive not seen many sandboxes which work well with grinding. Not to mention that sandboxes are not unlimited property. If you dont want to add just 1 new Building at a time (which would defeat the sandbox idea and make it just a linear click and forget) you soon run into the end of the expansion cycle which means you need to add other additions to the sandbox (like upgrading buildings, upgrading other areas etc) which leads to more and more featurecreep (star citizien 2.0).
Just to mention something which is already in your game and has little to no air-/devtime (yet): My choices with the girls so far seem to have no impact outside of the direct scenes themself. So it does not matter if Morgana has 35 Lust or 5 if i click in this particular scene the lust option. The same applies to our hero himself. I made my choices but warrior or seducer give no additional benefit and often enough i wasnt even able to go a different route, i.e. i am not even able to go full warrior in every conversation or full corruption with every girl. Adding a new (sandbox) feature ontop of that will in the longrun mean that you will have very little time to work on these other existing features because there is only so much time "behind the lewds" you can work on in every update and sandbox is especially development heavy since it needs constant tweaking and readding of stuff to stay relevant. You might run in a situation where you simply abandon old systems because you find no time to come back to them, someting i personally would find sad since you initially thought of various paths and choices in your base design.
I hope this input helps and you can find some solutions to it, i have read above that you want to create 2 versions running alongside each other and i do not think that this is the final solution for you. Afterall you closed origins down for the exact same reason. To keep 2 (at a later stage) independent from each other games updated simultaniously basically would mean 125-150% workload compared to your current updates for very little gain. Concentrating on a good, not grindy and not overbearing sandbox could be a way to appease everyone. Noone complains about changing rooms or small freeroam scenes in most games even if that is the same thing as a sandbox just under a different label.