I hope it works out for you and you don't hate me at the end of the summer.
My garden spots are only about 6 inches of compost and sandy loam with red clay underneath .
In my experience it is more important to have plenty of room for the roots at the 6 inch and up level than down deep.
So the wider you can make that top layer the better off you will be.
You can even cut back on the wood mulch and add more compost and loam if you want.
I have also noticed that every now and then I have a plant the wilts when all of the other plants around it seem to do ok.
I would water the plant and it comes out of it, the only thing I can think of is some kind of critter has eaten some of the roots, ' like a grub worm.
Worth