This might sound silly, but have you considered to also backup to optical media like DVD-R or Blueray? To be clear, not as a replacement, but as an addition to the whole hoopla of backups, as I think optical media are more or less 100% resistant to ransomware. They’re also water resistant and most likely of no value to most thieves and burglars.
Apple surprisingly still sells the Superdrive, but any of such devices will probably work. Store the disks in the dark and replace them on time (which you’ll probably do anyway, as a read-only backup is obviously nothing more than a fixed snapshot of that moment in time). As with other backups (including cloud based solutions) you might consider encrypting them before writing.