First of all, I've worked in our holdup squad in the past, and I can't recall anybody committing robberies with an unloaded gun.
That said, I think the real issue is the level of danger that both offences pose, and mandatory minimums should be in fact that. In circumstances that are more aggravated, and certainly in the case that you first describe, I would be strongly of the opinion that a four-year mandatory minimum would be the absolute base that one would start at, but I would expect, with all those aggravating factors that you've outlined, that a judge does and should have the discretion to impose a much more significant sentence for a person responsible for multiple offences who clearly poses a significant public safety risk.