The version control is not a "backend" on fossil.  The version control, the
wiki, and the bug tracking are an integrated whole.  It is not feasible to
attach a separate "backend" for a different version control system like Git.

If you like the features of fossil, but are burdened by a Git legacy, I suggest
you work on improving Git by adding to it feature you discovered on fossil.
Failing that, consider writing a tool that will import your git legacy into
a new fossil repository.