Now this is something of interest. Did you take a big light with you? Camera phones do well with lots of lights but perform badly if you use their flashes when doing Urbex.
I did a factory tonight, and will be posting it soon. This is one of the subjects I write about (when I have the material). Gaining access can be challenging.. and you never know whats gonna happen.
Access is easier when someone from the historical society unlocks the door and guides you through. Natural light and the pitiful phone flash were my only means of illumination. Some shots just didn't work out very well, like this one of a room with one uncovered window.
The top floor had more open windows, and more natural light.
I haven't enjoyed what you describe to me as a 'permission visit' yet. All mine are technically civil trespass, meaning the owner can turf you off at best and that's about it.
The thing is the owner's of 19th century mills are rarely about to do that so as long as I can gain access without vandalism it's generally OK.
If you do any more, I would recommend a huge light, like this:
The differences to your photography are massive, and even iphone's get the job done verywell.