Additional info:
Trying this with a bog-standard replicated pool
(ceph osd pool create reppool 2048 2048)
succeeds perfectly.
So, I assume the issue is the write-amplification from the ec pool implementation?
My current settings are k=10, m=2
Reading in the list, there are additional suggestions about setting k to powers-of-two
(which aren't well documented in the official docs...).