Happy holidays to you too!
That pinning feature shall accomplish that (I’ve made a bunch of edits to the post, they’re crossed in the mail)
That actually shall help too (and be much safer; I have horror stories to tell about sinology’s write cache implementation, and I doubt anything changed since) – you’ll have double space, and storagenode reads more than writes, so that shall still be helpful. with consistent load caching writes does not help much – they still have to end up on the disk, there is no reason to buffer in hopes for lower load in the future if the load is pretty consistent.
Exactly - I was going to write that you can keep this one as cold archival storage, but I’m so disappointed with synology that I don’t want give more reasons to keep using them 
On one hand, it may increase in data space consumption, because it does not preserve extent sharing. On the other – storagenode data is mostly small files that are not fragmented in the first place. They are scattered on disk, yes, but they are also randomly accessed, so piling them up in one corner won’t help much.
I’m not sure. I’ve never done it on synology (I pretended SHR does not exist) but I did read that not all configurations can scale. You may end up replacing all disks with the same size and not being able to utilize full size, because of the pre-existing mini-arrays that synology could not detangle.
I think in this case you likely will end up in generally same place. While order in how you added drives matters, and perhaps synology could do better when creating array of all disks at once, as opposed to incrementally, the very nature of SHR is to take advantage of disk size variety, so if disks are of different sizes – it will have to create multiple arrays. For example, if you have 10 6-TB disks, and 4 8-TB disks, creating SHR2 will result in one RAID6 array of 14x 6-TB disks and one RAID6 array of 4x 8-6=2TB “disks”. That latter array will be horrifically slow, so while you do have 14 disks to share IOPs indeed, when writing to that space you will experience RAID6 of 4 disks horribleness.
I can never write a coherent post in one go. Usually I try to fit in under 5 minutes before Discourse shows Edited flag.
Yes, I think this is manageable on a per-share (sub volume) level in synology. You can disable atime updates (definitely) and sync writes (probably) on the fly, but to disable checksumming you need to re-create the shares (perhaps not worth the trouble) because synology decided so. IN reality, btrfs allows to configure on a file granularity, but sinology understandably did not expose that complexity to the end users to minimize confusion.
