Hi there. Sorry for the slow follow-up on this - there was an issue that meant I didn't get the email alert when it was assigned to me.
There is increasing evidence that culling old / poor performing content from your site can have a positive effect, though I wouldn't be particularly confident that this would transfer across sub-domains to benefit the main site.
In general, I suspect that most effort expended here will be better-placed elsewhere, and so I would angle towards the least effort option.
I think that the "rightest" long-term answer though would be to move the best content to the main domain (with accompanying 301 redirects) and remove the remainder with 410 status codes. This should enable you to focus on the most valuable content and get the most benefit from the stuff that is valuable, while avoiding having to continue expending effort on the stuff that is no longer useful. The harder this is, though, the less I'd be inclined to do it - and would be more likely to consider just deindexing the lowest quality stuff and getting whatever benefit remains from the better content for as long as it is a net positive, with an eye to eventually removing it all.
Hope that helps - I don't think it's a super clear-cut situation unfortunately.