Well from one developer to another:
You chose a title that was far beyond your abilities to replicate, got disheartened when it got tough, and then quit. Development is difficult and in order to get ahead, you have to persist and keep at it. But, if your skills aren't going enough to accomplish the task, then what you need to do is either choose a simpler project, for your first project, or acquire the skills you need, and build the project then.
What you could have done better:
1] Not made a Patreon for it, if you didn't intend to complete the project. If you intend to complete it, then certainly seek out people interested in supporting you, but if not, then you might destroy your good will on the internet, which will make future projects very difficult to fund. Without money, its pretty hard to develop full time, and so if you do create another project make sure it isn't some hobby where you are just trying to learn stuff, but something you actually intend to build to completion. Your definition of completion and your player's definition may differ. Being open with people and transparent with your goals for the project will certainly soften feelings if nothing comes of it.
2] Having a grandiose vision isn't a bad thing, but you need to be able to temper it with reality, and most people with grandiose visions, can't do this. So if you can't figure out how to do this, you need to create a much smaller vision. Take Konashion, who with the first prototype of SDT was only really trying to emulate a basic flash game, with a slight amount of features to go with it. It was only after SDT was a hit, that they started to expand their vision and make it more grandiose over time, rather than start out with a huge vision.
3] Some people like deadlines, some like me aren't very good with deadlines. If the update is really good, it is often worth the wait, but with your project, very little changed between releases for months at a time. So if you are going for small incremental improvements, you need to release new builds more often. If you are taking time to do research, maybe put up a disclaimer that you are doing research and won't be doing releases for a while. If the game is too difficult for you to achieve, you need to tell people, and then tell people what you intend to do about it.
4] Giving updates on a more frequent basis. Doesn't have to be a new build, but knowing what you are working on, what you've researched, what you've balanced/tweaked, over the past month, would help to keep people's interest in the title. There was times with your Patreon, where there was at least 3 months of silence, and of course after the last release, no one heard anything from you in like a year, and everyone thought you had died or something.
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For the first three questions:
How would you achieve modding in UE4 then? Through the Toolkit, external files, unpacking the compressed data folder, etc? I'm interesting in modding UE4 stuff, and am curious how you'd achieve this.
1] Inverse kinematics tends to only be an issue with walking/running style of movement. Not sure the issue with a SDT style game though. Animations are tough for both 2d and 3d; they just have different kinds of problems. Hiring an animator wouldn't necessarily solve the problem, as they can only do great things with a great model. If the model was highly limited in bones, vertices, or textures, you may still have issues.
2] Fluid simulations in any game system or renderer are difficult. It would be interesting to know how True Facials managed to achieve it in Unity. YEAH, UNITY AMIRITE?
3] Most of the time, the part that gets the least attention in that case is the female genitals. Like for the longest time, in True Facials, they looked like total crap. And the big draw for a 3D camere is to stare at genitals. Yep.
Anyway, if you ever did intend to restart the project, you could instead work with the developer of Hyperdeep, another SDT clone that looks like it has promise...