If they don't do a boxset, I would hope that they re-release the first few.

Try HLJ and other Japanese sites, as they re-released the first two (Scrapmetal and Rampage) at the end of last year, and Longhaul is still on Amazon... leaving the last four, that haven't been released here yet (at least, not at proper retail), which means Amazon could be the best option for the last four as well.

Someone on one of the American fansites a few weeks ago claimed that a Hasbro rep or store listing said that there was a Boxset coming. I was told from Hasbro Australia that there wasn't one on the horizon, but that was about 5-6 months ago when Longhaul was just being released, so if it is coming, it wasn't on the schedule back then.
I want to try to ask again, but I've now bought duplicates of the first four (based on that info last year), so it won't matter to me, but it might help others still on the fence.
(and I'll recommend Hasbro America re-releases the first couple later, before Overload is out at the end of this year)