I guess I have 3 questions/concerns/opinions:
1. I really feel children learn best by doing and they need to be using manipulatives and understanding basic concepts that way before doing any journaling or worksheets to gain real understanding.
2. Maybe your group is different than mine, but many of mine will start the year not being able to do what you are asking. They don't have the fine motor skills or experience. Drawing a triangle, for example, is hard for many children this age! Many don't have the representational drawing skills or writing skills to do what you ask and I am afraid they will feel like failures. At least with open-ended journals they can feel proud of whatever they drew, or represented, depending upon the stage they are in, and can grow from there, rather than feeling they aren't capable of the task.
3. I don't understand the way you seem to do themes. Why isn't there time for basics like math? For examples in an ocean theme you can have them sort shells, count shells, pattern with shells, etc. etc. Also, just because you are thematic doesn't mean every activity you do has to match the theme. You can and should still do all the basics - science, math, writing, motor skills, etc. whether or not it matches the theme.