@MvL Welcome to the Rockstor community. We have an existing thread that may help on the SMR question:
I think it comes down to if they are host or self managed disks but I don’t know. That thread and the links it contains may help with this question.
And on the btrfs RAID 5, 6 question we also have a rather inactive but still useful thread:
It might be worth waking that thread up if the info you find doesn’t answer you question. I do know that raid 5/6 balances are currently very much under performing time wise in comparison with raid 1 / and 10, ie taking days to complete on large file systems… If in any double I would suggest you go for raid 1 or 10 for the time being.