I don’t know how relevant my comments will be, so please feel free to disregard.
I’m a “hobbyist” [wannabe pro 
], that loves playing with Elm. Started off teaching myself Flash in the Macromedia days, then Ruby/Rails/Javascript, and in the last few years Elixir/Phoenix/Elm. I’ve always enjoyed coding and learning.
I’ve never really taken to Slack, only used it once for help.
Personally I prefer the more formal approach of SO and Discourse. Mainly because, over the years, I’ve found that the formal approach forces me to think through a problem from the perspective of the potential reader - more so than I might do on a more relaxed chat channel such as Slack.
“I have this problem I can’t figure, and I need to explain it to someone who has no idea of the picture I have in my head.”
If I’m asking for help, I want to give the potential helper all the help I can to help me. Therefore, typing out and structuring the question in a way to accomplish this forces me to consider the problem from a different angle - when previously I might have been stuck staring down a tunnel and struggling to find another route.
So, I’ve found that going through this process has, on a number of occasions, helped me find a solution myself, and I don’t end up posting the question.
When I do post a question, I like to be able to bookmark the answer, no matter how trivial the question may seem. It may be a problem I only encounter every now and again, and if I was to ask on Slack, I wouldn’t be able to bookmark it so that I can go back to it the next time it’s a problem.
This question I posted is one I’ve gone back to a few times: Json payload now traces out as <internals>
It’s a simple question that could easily have been asked and answered on Slack, but the fact that it’s a permanent resource on Discourse has made my life simpler on a few occasions over the last year. I’ve revisited it enough times now that I shouldn’t need to again, but if I’m having a thick head day, it’s there should I need it.
As a final thought, I’m not worried about posting a ‘dumb’ question. If I get to the point of posting, it’s because I’ve gone down all the research roads I can to find a solution, so if/when I get to that point, I wouldn’t consider it to be a dumb question - it’s just something I’m struggling with, and this community is friendly enough for me to feel comfortable posting what others may think is ‘dumb’.
I guess there is a place for Slack, but my preference is likely to be Discourse for the foreseeable future.