First off, beautiful suggestions, they all make sense and would work.
You can put the full description in your about the category post and it as Jay said, only the first paragraph would show which helps with the design you mentioned.
Then pinning the topic or any other topic only pins the first original post IIRC, so you’re good there.
You could also run tests with these designs to see how they look so you can confirm, I am currently trying to design my own forums too and I keep testing the ideas that come up for me.