A French train ticketing site called capitainetrain.com is really rather good. Have a look for yourself. It's better than some of the train ticketing sites native to the UK. I was impressed. Sign up login and go. See if you like it better than what you’ve been using.
How do you solve a problem like how to save money with Eurostar when you haven’t got all your dates confirmed? I found a solution that will buy you a week of time while you finalise all the other parts of the trip. This post tells you what I found.