A few observations, but nothing super actionable unfortunately. You would probably do well to engage an SEO specialist.
- It’s a fairly recent move, so things might just need some time
- Looking at your charts, somehow the total traffic (which is what I’m assuming is the second chart) seems to be very steady, which is very strange if you’ve just moved a significant chunk of content away from that total.
- The content is in the new index, including fairly old topics. So the indexing isn’t the issue.
- The old content is still in the google index, which is sort of surprising, but might be due to reason #1
- Could this just be an issue with the search console? Do you also see a decline in e.g. visits and users posting?
- There isn’t a lot of activity. Looking at /latest there is no content in e.g. October. Are you sure that all relevant content is accessible to google?
- Did you modify your robots.txt in any way? if so, consider reversing that to defaults