Have you tried to tune a new write cache/aggregation option introduced in Storj v.1.4. ?
There was already write caching before, and after v.1.4 it is available for user tuning via config files or command line option. It was disused about month ago in the corresponding GitHub issue: https://github.com/storj/storj/issues/3854#issuecomment-624522307
But it looks like that nobody of the SNOs with SMR HDDs has yet been tested it. You can be the first one.
Default value for write buffer is 128KiB (per each data piece/upload). It is interesting to see if increasing it to say 1-2 MiB will help mitigate SMR performance issues.
filestore.write-buffer-size: 1MiB