I think you need to set strict_ordering=True to get backfill. The number of top jobs is then set with the server or queue backfill_depth parameter. See “Fairshare and Large Jobs” on this forum.
Also, I think your job_sort_keys will be ignored if fairshare is turned on, unless the fairshare entities are equal. See “Fairshare with queue priority” on this forum.
We use fairshare with queue priority at our site, and it seems to be behaving as we expect.
Cheers,
Peter