Hi James,
Great that you've checked out those items and there aren't errors.
I'd break my response into bullet points so its easier to respond
1- I'm bugged that the traffic loss occurs in the same month as the https redirection.
That completely tells me that you've either killed, redirected or noindexed some pages that drove a lot of traffic.
2- Also it could be possible that you didn't deserve that much traffic due to either being ranked on searches that you weren't relevant or Google didn't fully understand your site. That often happens when migration takes places, as Google needs to re-calculate and fully understand the new site.
3- If you have still on the old HTTP search Console property, I'd check as many (and in some scalable way) keywords as possible, trying to find which have fallen out in rankings.
4- When checking those keywords, compare URLs that were ranked, there could be some changes.
5- And lastly, have you made sure that there aren't any indexation and/or Crawlability issues? Check the raw number of indexable URLs and compare it with the number that Search Console shows in the index coverage report.
Best wishes.
GR