Is there a way to spoiler part of your post?

You can edit your own post for a short period of time after posting it, but after that it is probably just better to reply to your own post to explain what you've figured out and what you'd still like responses on.

So that is a confirmation that we cannot spoiler text?

I want to update my post at the top instead of just make a whole new post and spam the board

Rather than hide it, use strikethrough text.

Use rich formatting > Format > formats > strikethrough

 Edit: You should do this anyway, because if you were to hide your text, any replies referencing the hidden text would look weird and confusing.

I've edited my own posts long after I've posted them, based on Eos software changes.