I feel you here. And I have seen how much those 3rd party communities can be catastrophic for some products.
I believe here the same logic applies when we are talking with prospective customers who are worried about competition between their official forums and those other options: by owning both the product and the community fully, you have a lot of leverage of other communities.
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You can give your real customers perks on the forum, like titles, badges, exclusive access to categories, etc.
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People want correct answers to their questions, and having a direct line to the people creating the product is invaluable.
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You can reduce friction by integrating login between Discourse and your product login.
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You can post announcements first in the forum.
Reddit is a great link aggregation and news website that I use myself a lot, but if your engineers are replying only on Discourse why would people go ask for help in a subreddit?
As for a Slack, why would I spend time replying to the same question over and over in chat, if I can reply once in Discourse, have the question be indexed on the internet, easy ways to merge duplicates, refer old discussions in new features, etc.