You should see this vídeo. https://youtu.be/8OHpKotJ3yQ It explains in a excellent way how to use ている. That channel explains a lot with good examples.

Not a native speaker, but nobody else has jumped in, so here's my understanding of it. Take it with a grain of salt because I'm not a native speaker. If you want I could ask my sensei for clarifications, I just would prefer to not bother her.

These two phrases only have a different nuance, not technically a different meaning. The nuance is exactly what you described in what you consciously know. So you might use the 〜ていた form to say how it was safe for you to go on a hike, and the 〜た form to focus on the season change itself. It's not necessarily wrong to use them interchangeably.

I mean, in terms of nuance and meaning, you’ve pretty much got them figured out. Your translations correspond with the way I understand those forms as well, so yeah, 〜た/〜る emphasises the certainty of an act, while 〜ていた/〜ている emphasises the state. If you’re struggling to remember when to use which or you think you mix them up sometimes, maybe you need to practice more listening and reading, and repeating along. That will help you sort of internalise what each means and how it’s used.