I have been using SMS Backup and Restore Pro for years. Since I multiboot on most of my devices the app gets a lot of use. Don’t believe it has ever failed to do the job regarding SMS (don’t backup call logs since I rarely make or receive calls). A must-have app in my opinion.
Nice to see it get a write-up. 
Only issue I ever had, not the app’s fault, was with some OEM stock ROMs. When messages were restored the system would automatically give MMS messages (with images) the current time stamp. Totally messing up threading. That was a long time ago though.
All but one of the devices have external storage which I use for the app’s backup location. The Essential PH-1 with /e/OS only has internal storage. Many thanks @smu44 for the Documents subfolder tip. Excellent.
EDIT: Just found out that /e/ does SMS syncing now. Nice.