I think it is a good idea to have a ditch kit and if it is not on you it won’t be with you.  Looking at your plans I have 2 comments.
1      TOO much stuff in your ditch kit.
2      A separate backpack to be worn over your backpack is a pain.  If you can make it part of your PFD then putting it on is automatic.  I took a small fanny pack cut the straps off and sowed it to the back of my PFD collar.   Among other things my SPOT messenger goes in there.
About ¾ the way down this page is a picture of my back where you can see the fanny pack.
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