For a while, I used Walling after ditching Notion in 2018. In 2020, I started using alternatives like Craft and Taskade instead.
More recently, I tried out Microsoft Loop and ended up sticking with it. It’s very good! 
Circling back to Discourse though, I don’t think it’s trying to replace Notion or any of the alternatives I’ve mentioned above. Notion might work well in tandem with Discourse, but it certainly isn’t a replacement of any kind.
I’d be interested to know if anybody uses Discourse alongside Notion. How has it benefited you? Do you feel more productive?