Which file system for an SD card permanently mounted on my Mac?
I tried for myself, formatted the SD card in GUID + HFS+, and after several months it seems more reliable than exFAT for this use case. 

When the  card is accidentally disconnected the OS shows a notification, but as I unplug and replug the card, the system checks it for defects, and run fsck in the background if needed (it happens rarely, and it takes less than a minute). 

So this setup it's a good solution from my point of view.

It's worth remembering that you should use it only for static files, i.e. data that you read often, but write seldom.