Your AXS batteries dying mid-ride is unlikely. A fully charged battery will last hours upon ride hours, depending on the component:
Road derailleurs: 60 hours
MTB derailleurs: 25 hours
Dropper posts: 60 hours
Suspension: 20-35 hours
That said, a spare AXS battery weighs 24g and will be hardily noticeable in your tool roll, so why not carry a spare for your long ride?