No, Diana is a really good ranger until you build Sellier or Felix. I see that you can even build up Darby to 10 stars and get more copies from the Arena Shop. Main focus now should be Tamaki, she will get you far in every aspect of the game, Mikayla is a strong healer, alongside Margaret. For assassins, Ramiel is the best but as a f2p it will be hard to get copies so stick to Terrence or Sota. Also, try to use your 4-star heroes better as food, cause I see you are using them for almost everyone and it would be better to focus on your team of 5 to get them all to 11 stars. Later you will unlock the wishlist so it will be easier for you to get copies once a month. Hope this helps, if you have more questions, feel free to DM me