You're already doing great with lots of clicks and impressions already. My google analytics / webmaster tools doesn't look like that tho hmmm... In comparison for a 28 day period I have 20k clicks, 7.6% CTR and Average Position 14.1. We have 40k posts, 5k threads, 88k users.
Forums are typically filled with "thin content", meaning pages with little actual text content. People are searching to find information, if the pages have little information they don't have value to the user and google won't rank them well. The best way to increase page rank and therefore CTR is to increase the amount of copy (text) on your board.
You don't have to reinvent the wheel tho. I've had good results with this method. Re-read the "good" part, keep expectations realistic.
Find your top 50 top landing pages/threads
Sort them by highest position and look at the top 25
These are pages that are doing ok but not great and will be easy to capitalize on.
Every day, pick one of these threads and do a little on page SEO. Checkout what keywords bring people to it. Focus on long keywords (like a phrase instead of a word) and unique keywords for your content. Make sure the keywords are in the thread title and once or twice sprinkled throughout the thread. If you can somehow put 2 different long chain keywords in the title, that's even better. Add pictures, format the text nicely with bolding and paragraphs. Implode the content, meaning add more text/information to it. Focus on tutorials and guides. Or turn a big thread with multiple pages into a guide by combining all the best content into one post and moving it to the beginning of the thread. Be very careful with keyword stuffing, seriously don't use the same keyword more than a 2 times in a small post, or 3-4 times in a large post you will get de-ranked by google and it will be very painful. People use many different searched to get to your landing page, you can rank for several long chain keywords instead of the same one over and over again and avoid keyword stuffing.
I don't know how much time you have but if you can commit 1 hour a day and take your the bottom half of your 50 best pages and turn them from rank 15 on the second page to rank 5 on the first page of google, you will see a good improvement.
Depending on the topic of your board, you can contract people on Fiverr to write stuff for you for cheap, just make sure it's decent.
I'm no expert but I've done my research and find this to be a "good" way to do SEO.
