If you're going to use artificial means to make her comply, then it needs to have negative consequences later in game. For instance, the chronic use of alcohol or sleeping pills on Ellie makes her dependent upon them, and damages your relationship toward the end game. You may also look at the possibility of creating different forks for Ellie and MC's relationship, such as a romance route, which is slower, but makes her more willing to do things with the MC voluntarily; the drugged / drinking route, which makes her do things faster, but ends up either killing her, or causes her to kill MC in the end if relationship points are too low; or the BDSM route (a fork of the spanking actions) that allows the character to building some interesting devices to use on Ellie, but has some drawbacks, as well. I'm not telling you how to make YOUR game, but just throwing some ideas at you... and possibly adding to your workload.
I think that her face is a little too androgynous at the moment. A little too boyish. I'm not sure what to tell you on fixing it, but take her "age" into account and search for something appropriate.
I like your ideas, but when I was thinking about creating this game, I wanted to avoid creating a game by the "standard template" like a fixed love or slave route. I want to mix things up and make it more "alive" and to add a possibility for her to feel "love", be perverted or not, fell sexual desire and so on. So for example the spanking event would lower her love for the mc and would rise her lust level. And different events in the game would have different consequences, so there would be no "fixed" routs and their relationship would be more dynamic.
As for "killing her" it's quiet dark
I don't think, that I'm going to do such an option in the game, but who knows.
About BDSM and "devices", I've added the workbench mainly because of that. And I also want to create the basement in the house, where you could build such devices. But at first I'll need to know if it's a popular fetish people would want in this game.
I'm also thinking about the option of capturing enemies in the game, when the fighting system will be implemented.
So it would be possible to capture an enemy in the basement and have fun with her. But I don't know if it is allowed on the patreon as it is "slavery".
I understand that you are not telling me how to create my game
And I will only encourage everyone to share their ideas like you do as it would help to create a really interesting game, for everyone to enjoy, so thank you for your ideas. Fell free to share them as much as you want!