We ended up replacing the Pi(s) by an Intel NUC, which was just over 100 Euro in 2018 without memory and storage. For relatively low cost, you get SATA speeds (or NVMe if you are willing to spend a bit more) and an AES-NI capable CPU. Our current NUC has been humming along for 4 years.
Ps. Syncthing is not backup, even with a read-only node. You are just one Syncthing bug or sysadmin failure away from erasing your data. Having the files on btrts on a single SSD only makes it worse.
Just get some redundant block storage somewhere that supports object lock (to prevent accidental deletion or ransomware encrypting all your files) and do incremental backups with something like Arq or Restic. There are some good services where 100GB storage costs $1 monthly.