Keep your RAID-1. It will protect against single disk failure and improve read performance.
Build a server. Just a little one. An old desktop PC will suffice.
Put in a cheap SSD (64GB will do), and a 3 or 4 TB HDD.
Install your OS of choice to the SSD (Debian, BSD, anything low maintenance).
Write or procure a script on the server to rsync the contents of your desktop PC to the large HDD, with the --backup switch, rotating monthly. This way you get overwritten or deleted content put in another folder (usually the day of the month), so you get one month to recover accidentally deleted or overwritten files.
Set this script such that it starts ten minutes after the server boots up, waits another ten minutes, then powers down.
Set the server to Wake on Lan.
Have your desktop PC (or other server if you have one) send a WoL magic packet to the server once a day.
Turn off the server.
If you ever need to retrieve anything, send a WoL packet to the server, wait for bootup, log in and kill the backup script (which should still be sleeping if you do it within 10 minutes).
Voila, cheap robust offline backup.