Questions before install OpenWRT on Zyxel NSA 325v2

I would like to install OpenWRT on my Zyxel NSA 325v2. I am a little unsure if this is a good idea. Can I access the existing data on the disks / partitions / shares again after the successful installation? There is some data on the hard drives that I can not easily backup before.

Is it possible to set up the NSA 325v2 again with OpenWRT in the same way as with the original OS? Is there a web interface or is the configuration only possible via command line?

Thanks a lot!