The 2-3 posts a week aren't as difficult or time consuming as they seem. You could simply get information online or from a book and transfer it to the coven, giving credit where credit is due. Some covens don't even mind if the forum post itself is a question, not an informative post. Don't be short-sighted by the rules that seem constricting, covens are there to help people learn and grow!
Many of them will be fair and good to you, and it is a great experience for obtaining knowledge as well as getting to know others. I suggest simply trying your luck at whichever one instinctively attracts you.