I appreciate your concerns but this is just a unfortunate side effect of the game getting more complicated.
First of all there was no engine change, it's always been made in unity.
As more content gets added to the game, with more complicated visual flair it gets harder to run and optimise for lower end android devices. I'm not an android developer myself, I barely know how to make the game run on PC. But I should be clear that this game is design WITH PC IN MIND. I can't lower the quality of the game to support random Oppo phones with 1gb of ram, if I was better at programming I might, but I'm simply just still learning.
The save reset was in service of making the game run better and the game's running better than ever on PC, at least without many major bugs showing up. It's just something that has to happen in a game that's constantly being updated, especially on a complicated engine like Unity. I often regret not making the game in Renpy because at least Renpy's saves are consistantly usable for the most part. But it's too late to port to Renpy I think.
The content skip option is there specifically for this reason, I give you acces to basically the full gallery at the click of a button, and progress right up to the most recent update I don't know how else I could help you. (Also the next public update will have another save reset so feel free to get mad at that too, but i'd reccomend you just use the in game features that bypass any trouble.)
The settings (on android) don't change much because I have to force the videos to be lower quality for it to even run properly. It's just not my realm of expertise and having hundreds of animations, especially with how many there are for the new sex events, it's just not feasible to expect them to run on just any device. Not with Unity.
I am commited to the story of Dimension 69, I can't really abandon it for a sequel, that wouldn't make any sense.
I assure you that on every phone and computer I've tested the game on, it runs completely fine without any lag, so I can't really relate to performance issues but I empathise. I just honestly don't have the knowledge at hand to do anything about it. So, sorry.
Bottom line is, any technical changes I make to the game are to make it run better and look better (ON PC). If your device can no longer handle the game, I apologise, if I knew a better way to compress these videos or make a port of the game to fit that device, I would. But I don't.