Shifting a review process from one journal to another journal
At this point, it will not be fair to the reviewers that are currently refereeing your paper to receive a notification that the paper has been withdrawn. 

Although you might think they are not doing their job properly, reviewers are busy persons and a lot of things can happen in the between. Given that 5 months is not that much time even fr a revision (at least depends on the fields. I my field of applied math/TCS it's OK) I would suggest to wait a little bit more but also bring the issue to the handling editor (which you already did).

On the other hand, even if you decide to withdraw, which is your right, I think that supplementing the current reviews to another journal is way off and I would strongly recommend against that. For start, the new journal would have no idea who is behind that reviews, not to mention the ethical levels of such action.