I deliberately ignored the timing part of your proposal, and instead
think in "stages" - here is a plan I find sensible:
* maybe you make an test packaging of 3.1.5 - not uploaded to Debian at
all
* if wxWidgets 3.2 is in Debian when you are ready to upload to Debian,
then have your package link against that
* if not, then you approach the wxWidgets maintainer about creating a
package for it, which would stay in experimental until no longer
experimental (i.e. when either ABI has stabilized or packaging
contains reasonable tracking of unstable ABI e.g. using symbols
tracking).
* if wxWidgets maintainer find it unreasonable to package wxWidgets 3.2
before its ABI has stabilized, you might consider embedding a snapshot
in your own package - released only to Debian experimental.
* of there is still no wxWidgets 3.2 package in Debian when Debian gets
close to freeze **AND YOU ARE FULLY WILLING TO MAINTAIN YOUR FORK OF
WXWIDGETS YOURSELF FOR SEVERAL YEARS**, then release your experimental
package to Debian unstable.
* otherwise wait until wxWidgets maintainer consider it reasonable to
provide the needed version of the wxWidgets library.