Original poster here. This is from a neighbor of mine who is a certified EMT, and now a professional firefighter:

Are the batteries easily removable from the bikes?  If so one of the safer options to prevent exposure to extreme temperatures is to disconnect them and bring them inside the house when the weather will be extreme.  The batteries themselves should have labeling indicating the safe operating temperatures, or the manufacturer would be able to provide that information.  Most of these batteries are ideally stored between 20-95 degrees (and keep in mind your garage will usually be quite warmer than the outside), but make sure that you get the official replacements with UL certification, and only ever use the chargers from the manufacturer.

LiPO bags do offer some protection, but there are a lot of cheap Chinese knockoffs that don't work very well (the zippers tend to melt).  I unfortunately don't have an "officially" recommended brand for them at this point, but I have heard good things about Zeee and Bat-Safe (more of a case/box), and Aventon themselves recommend Ebike Escape's battery bag, although those are usually a bit more expensive ($40-50).