You could wait till April to send the letter. By that time, you will have done your revisits and have a pretty good idea. Also, admission offices do not start working off the waitlists until all acceptances have come in. Therefore, the fact that you send the decline letter early does not change anything.
In terms of burning bridges, I would not worry too much, either. Once upon a time, my cousin turned down an offer from a wonderful school because they believed that another school would be a better fit for their Kiddo1. 2 Years later, this school still accepted their Kiddo2. Bottom line: as long as you phrase it nicely, the admission folks will understand and take it well. Handling rejections is part of their job, after all.