Whatever you are able to do for your daughter at home will have tremendous value for her. She probably needs that encouragement and lack of pressure. If Teaching Textbooks works for her, then stay with it. I am not familiar with that program.
Slow and steady is key. You want to work with her strengths while encouraging her to move at her own pace through those areas that she struggles with.
I suggest using audiobooks during the summer. You can find free audiobooks at your local public library and online at Librivox.org. I also suggest reading to her during the summer if you are not already doing so. Choose books that are above her reading level. Even though she may struggle with reading, she can still "keep up" with grade level books through audiobooks and being read to.
You indicate that she loves art. Can you enroll her in a drawing program to build this interest? The observation and work with spatial dimensions will be helpful for both a processing disorder and dyslexia. She can learn how to cope with these challenges through art depending upon the level of the dyslexia and the level and type of processing disorder. Art is not a cure-all, but it can help, often by giving children a creative outlet and the freedom to experiment with the way they view the world.
During the summer, try educational games and puzzles. These can be purchased at most educational supply stores. You can also create your own games and play old games, such as Scrabble and Yahtzee. The following site has instructions for paper and pencil games: http://www.pencilandpapergames.com/. The idea behind playing games is that they are fun and yet build skills without the pressure of a grade. Many of them rely partly on luck and partly skill which evens the playing field.
I hope this information has been helpful. May God bless you and your family.