Armed with the information provided so graciously ... (
How about supplying your camera and exposure information so we do not have to look it up? If the first picture is merely the jpg of the raw, you are still a good two stops underexposed. What exposure mode are you using? If you are in manual, that might be the problem. Are you checking the viewer to make sure your exposure is good?
Raw files. You cannot make them smaller, brighter or anything else. Your software does that and then you share the results, typically as a jpg. You can share the larger raw's by uploading them to something like dropbox. And, as Linda says, never, ever change the extension. That is a property of the file type and only software can change file type and, by extension, the extension. Excuse the pun.
As for the polarizer, it works when you have bright, direct sun which is, optimally, parallel to the sensor plane, and when rotated to maximize the polarization. When all three of these conditions are met, then the filter works. Otherwise, you just lost about two stops of exposure. It also works better at mid-day than in the golden hours. From this shot, I cannot tell if using it was appropriate.
People share the misconception about processing raw's as you stated here. The only difference you really notice is that you have more exposure latitude and better colors than with jpg's. Otherwise, you do all the same things but with better results. I missed one other thing. You share the results of editing raw's by exporting them while the actual raw remains completely unaltered.
Final issue. Having a longer lens would help a lot. To get your final picture, you cropped a lot and that degrades image quality. I know cropping is cheaper than buying a new lens and also a lot lighter but you might want to add something like a 150-600 to your wish list. I did not look to see what lens you used here so this comment might be irrelevant. Also, I suggest using a tripod and an aperture two stops down from wide open.
I hope these comments help and you will go out there again for another try.