I really like this! It sounds so cool and majestic! It really suits a fantastical setting.
I like the intro and I think you're almost there with it. Just a question, the 2nd theme starts at bar 14 right?
Either ways for dynamics, I think the bit you have as f works really well but in those empty bars at 23, to contrast, you could have a p section with a less polarising texture, and then jump back into the big stuff.
When it comes to dynamics, I personally try to go for what I want the players/audience to be feeling and just what feels natural. If nothing other than forte feels natural, then it's probably meant to be forte :)