Don't buy books/video/music on physical media unless it's hard/impossible to get a digital version. But also don't rely on IP subscription services either. The Cloud is great as part of a backup strategy: but not as an exclusive service that could gate your access to your content.
Digital storage is great because it can hold anything: books, shows, games, whatever. And it can be easily copied, and sent around the world. Have some space you own: redundant and automatically backed-up to a Cloud service... then enjoy it for years. It will feed your ebook readers and media players and homelab devices for a long time, and take up almost no space.
It will work as you describe with no issues: vibration won't be a problem. Use large/slow case fans (and even for the CPU consider a 92mm model if 120mm won't fit) and it should serve you well for years. Good luck!
There's not a lot of demand for hardware RAID for only four drives. Hardware RAID in general is becoming less popular, and since many people can just stuff four more drives into an existing case... stuff like USB enclosures are often seen as "good enough' (though options do exist)
With options like ZFS being super popular in the enterprise (and homelab), what are your concerns with software RAID? I find it offers more flexible recovery options, more config options in general, works with SSDs better, and is faster.
You chose the least reliable way to combine drives, with the cheapest components you could purchase, routed through the slowest interface for SSDs. So I wouldn't be worrying about micro-optimizations.
You've got automated 321 backups right? ;)
For $300 start with a mirror of 14TBs, and expand to RAID5/Z/6/Z2 when you can afford to.