I'd recommend you to reconsider going the all-Doxygen way. Even though writing elaborate documentation using Doxygen may seem cumbersome at first, keeping two Documentations consistent will be more work in the long run. Apart from the added workload you have by combining the output products of both.
Doxygen makes it possible to include Markdown files as pages into your documentation (see this section of the manual that) and to externalize elaborate documentation in order to not blow up your source files too much (see this other section of the manual).
I should also add that I have had no previous first-hand experience with Docbook, so that might bias me a bit towards Doxygen.