We seem to be seeing the 504s begin at 16:54:27 which seems to be the start of the snowball.
None of the content changes leading up to that seem particularly remarkable for the amount of subscription contents they generated:
irb(main):086:0> pp SubscriptionContent.where(content_change_id: 21300..21335, digest_run_subscriber_id: nil).joins(:content_change).group(:content_change_id, "content_changes.content_id", "content_changes.created_at").order("content_changes.created_at").count
{[21300, "25000916-b720-4734-ada9-5e132cfb531f", Thu, 08 Feb 2018 16:19:57 UTC +00:00]=>128,
[21301, "c28088ef-056f-49f3-932f-8d52e2fc6094", Thu, 08 Feb 2018 16:21:52 UTC +00:00]=>128,