In the backup world, 50TB isn’t really a lot, and you’re not really ready to talk about tape systems or maintaining an always-on disk system. Also, HDD’s have been getting more expensive due to AI idiocy. But, cheapest is probably a second raid system, like 4x 20tb drives. Do the backup at home and then move the backup system off site and either keep it spinning, or make sure to spin up and test the individual drives every so often.