It may very well be that your game is out of the scope of Construct 2. 7000 images is a LOT for this engine. I've been struggling with preview times recently and I have maybe ~750 images.
If most of your images are small, and all part of long animations, you may fare better on export as they will be packed into spritesheets. This doesn't help with the preview issue, however.
I believe Aurel did something like you mentioned, having multiple project files which you combine from time to time.
Ultimately HTML5 with its automated loading/unloading may not be the best choice for a project like this - some fine grained control over what's loaded/unloaded could go very far.