Also, re. SHR, it's not a proprietary Synology thing, it's just their custom implementation of some of the features of mdadm (e.g. linux software RAID) that allows you to mix disks of different sizes. It's not hard to roll it yourself on a non-Synology system, it's just DSM does the heavy-lifting of setting it up for you.
FYI, SHR support is still in DSM 7: you can add a set of disks from a DSM6 device and it will use them, you just can't create new SHR volumes.