I guess it all boils down to what form of backup you are after. If its simply having duplicates of the data so that should the main location fail or you accidentally delete a file then using an external drive, be that attached to the network, or plugged in when needed is OK. If the data is more valuable then some form of remote back up is needed. These days you can get USB sticks that have 512GB or more capacity so you could always get one of these and keep that in your car. However a backup is only as good as the last time the data was backed up, so if you forget to bring it in and run the backup software then it somewhat defeats the object.
As for software, have a look at Macrim reflect
https://www.macrium.com/reflectfree the free version is ideal for making drive images, both full, incremental or differential