The performance of syncthing depends indeed on a few hardware capabilities. It uses SSL, SHA256 which are cpu/hardware intensive unless hardware offloaded. You would benefit from a good board with native ethernet (compared to shared USB ethernet on raspberries).
As you mention the ARMv8 CPUs indeed support SHA256 hardware offloading instead of CPU calculation but they are not cheap.
Keep in mind you will need approximate 50-100 mbyte of RAM per syncthing instance.
Also syncthing is NOT an backup system, it is a synchronisation tool but can be used in a backup-like way. E.g: sync to a central point and run a backup cronjob (suggestions are borgbackup or restic). When using multiple peers which do bidirectional sync to and from a central server you can get problems and having backup snapshots is not a bad thing then.
Good luck!