Hi Erik,
This is a major branching point in your career in photography, more important than any lens or camera upgrade. You really have to trust me on this. Not taking pictures in RAW is like not eating breakfast and lunch and then trying to run a marathon. It's not possible! There simply will not be enough reserve in your system. This is what's happening to your B&W. Here, unlike the previous image, the blacks are simply absent and the brightest whites are once again blown. The only way to deal with this without changing the natural lighting, is to wait until later in the day when the light is less bright and then your exposure might cover the shadows to the brights. Still in order to actually record all this information, you need to shoot RAW. There's simply no way around it when you are using natural light or high key images. When you use RW, the bright areas can be recovered and the detail in the shadows can be rescued up to 1.5 to 2 stops at each end in some cameras.
Hard drives are just $90 for 1 Terrabyte. If you go through your images and delete images that are no good then you will get rid of at least 50% of the space and have room for RAW files. I'm always astonished by how much recovery is possible using RAW files. Even better, RAW translation software is getting so much better each year.
When you consider the price of gasoline to travel to the place you have to take pictures, then that's more than the storage of RAW files. Just keep the best.
Even if you don't know how to work in RAW, you can learn it. We had a 90 year old lady with a Canon G4/5 camera and she shot in RAw and processed the images in Lightzone! Her name was Mary bull. You are much younger, my friend. And as stubborn as you can be, this is one change you just need to make.
You don't need more than one person to comment on your pictures if they know what they are looking at. Those who might think your pictures are great right now will likely be blown away by the improvements you can easily make.
Do you have a tripod? Even if you don't, likely your Pentax has a built in bracketing funtion. Set the self timer to 10 seconds and have the camera resting on a table or ladder or whatever is handy and let it take 3 exposure bracketed pictures for you which you can fuse together in so many different software choices to span the high dynamic range from the brightest light to the darkest shade.
Do I like your picture? Yes, very much, I do enjoy images of forgotten rooms. They allow my imagination to conjure up parties and lovers and fights and pretenses that went on year after year.
However, if I don't say this is not good enough for you potential work product, you have to listen and not be insulted. Your Pentax, as it is with the lenses you have, can take you all the way to the best jobs in photography. The block is how hard you will apply yourself to discard bad pictures and put in the extra labor to deliver on a great idea. Look at the trouble you went to getting this wonderful project going! If you have the right files, you can deliver the best prints. Otherwise, your pictures will be just below what's needed.
Asher