Is there a difference between learning math from scratch through the grade levels or topics?

The way you are learning math is completely fine and should give you a general overview of each topic. What I have found is that, by learning it through grade level, there is a smoother transition between the topics. For example, if you are needed to refresh your memory on topics like fractions and decimals, the topics you chose might not cover those as in depth as the grade levels would.

My suggestion is that you stick with the topics you chose for now. If you find yourself getting stuck on certain subjects, going to the grade levels could give you a more thorough explanation and help clarify some things. 

Good luck on your math journey! I hope that answered your question. Let me know if I can answer anything else.