DTM does have a few built in events for forums (focus, blur, submit, change). If you've already got direct call rules setup and they are allowing your to track the actions you'd like, you don't need to change to the default events. As for the rise in server calls, that is a side effect of tracking each action in the forum. A few ideas I have for reducing your volume of tracking calls:
Break the form into pages and send a single call for each page completed
If the form can't or doesn't make since to be broken into pages, you could fire a beacon for each group completed (fire one beacon for each group of information rather than each field).
Write a script that increments as each field is completed/validated and send that beacon before the user leaves the page.