I'm about to go crazy, I can't figure this out.

As in, the source image itself? You really can't, scaling up images will always result in quality loss, especially noticeable on text.

I mean in captions. The stuff I upload normally have really high resolutions, and other posts with the same resolutions don't have blurry resized text, and I don't know how to do that.

Not sure I follow. Did you mean the text looks blurry in the smaller view of the Hub, but OK in the larger version?

Yes, this 100%.

Then my best advise is to just see what the text looks like with the image scaled down before posting, no fix beyond that afaik.

This isn't really going to be possible beyond what KinkyFinn said. Your text is being squashed into fewer pixels, so some loss of detail is unavoidable. I believe larger text with more straight lines (basically bigger text in all caps) will be more readable when scaled down.

Using a font with thinner letters would help I'd imagine, based on how I understand scaling. Following the same logic, avoiding bold characters would be sensible, and if emphasis is necessary, to use italics or an increased font size instead. Not any sort of expert so take that with a grain of salt.

The bigger your image is, the bigger the text should be. All you have to do is add more space for the text if you have a large font and a lot of words.