I wrote a plugin that solves this problem somewhat. At least when it comes to permissions: Here is an introduction of how it works: https://www.youtube.com/watch?v=C5paAvTYty4
It connects the permission systems of both platflorms.
I think a good idea would be to take this and flip it on its head. Instead of pointing people from discord to discourse we could treat the conversations that happen on discord as an asset. Its a giant treasure trove of content that can be used to automatically create threads in discourse.
When I wrote the plugin to sync discord roles with discourse groups I recognized how powerful the discord api is. We can react to any event that happens on the discord server. So for example we could create discourse threads every time somebody uses a question mark. I think questions and their answers are probably the most valuable low hanging fruit on the discord side that we could pick and put into the discourse fruit basket. This would also widen the funnel into the community because suddenly the most valuable parts of the discord conversations become accessible from the normal web. Discord conversations are not indexed by search engines at the moment, so this could open up a big opportunity.