I’m duplicating my server hardware and moving the second set off site. I want to keep the data live since the whole system will be load balanced with my on site system. I’ve contemplated tools like syncthing to make a 1 to 1 copy of the data to NAS B but i know there has to be a better way. What have you used successfully?
Rsync and rclone are the best options as mentioned in other comments. If you want to get real-time with it, and the previous cron-based solutions aren’t what you want, look at the myriad of FOSS distributed filesystems out there. Plenty of live filesystems you can run on any Linux-based storage system.
I think the better question would be: what are you trying to achieve? Live replica set of all data in two places at the same time, or a solid backup of your data you can swap to if needed? I’d recommend the rsync/rclone route, and VPN from the primary data set whenever you need, with the safety of having your standby ready to swap out to whenever needed if the primary fails.