Some thoughts:

1. You need to get to the point where the slack is self-sustaining. This means that some members find it worthwhile to post interesting content, and it's enough content for other people to routinely come back to. This usually means that you need some cluster of people (3-5 will do) who really prefer talking on this platform. If a Slack goes for a few weeks with little conversation, people might leave; and that might discourage others from posting, leading to a bad cycle.
2. It's work to create one of these communities. However, if the work could be given to someone whose time is relatively inexpensive (or done quickly), then it might be worthwhile to try out more communities and see what happens. Worst case, they're a bit annoying for people to join, but it's easy enough to get rid of them.
3. I think there's a lot of potential value in connecting people into new communities, so I’m excited about things like this, but it’s also quite a bit of work to do a good job. 

I think if you can't find the space you are looking for you should create something (at least a low cost version) and then if someone tells you of an existing space that works, then you can inform the people who have already joined. 

Even if the space isn't particularly active it gives future organisers a starting space and potential people to contact who may be interested.

I think the main case where creating a space could be wrong is if the admin is bad at moderation and not open to improving the space. This also provides an incentive for creating spaces because if you don't, someone else could create that space which then gives a bad impression for others who wanted to get more involved.

I don't think it would be bad for there to be a workspace for each major cause/career area. It seems that there probably should be somewhere in between Facebook and the EA Forum for people to have discussions about causes they care about. I've written more about this here. Ideally the forum would be able to support sub forums, but it seems unlikely to happen soon.

I set up a Slack for groups that are smaller but still want to use it for discussion. At the moment it is mainly used by the EA & metascience subcommunity and sometimes by FIRE & EA. I thought it would be a good space whilst subcommunities are small to see if there is enough demand for their own space. If you want you could use channels on there to start groups you wanted to see.

Choosing the right online space can make a difference, especially if people you want to join don't already use the product you're suggesting. Different spaces also allow for different tools/culture/vibe, there is a brief overview of some pros/cons here but it will depend on your target audience.

Maybe there are also more general question to ask if you're thinking about coaching or nuclear risk sub communities, some of which may be here. If you think there should be an online discussion space, how does that fit into the wider ecosystem for the subcommunity.

In the past, I tried to create a forum where customers and small businesses shared ideas on what sustainability options customers wanted and would pay for. Seemed like a great idea in my head. 3 months later, everyone who tried it told me they found it interesting to read the ideas but no one wanted to post. 

I was banging my head against a wall when my mentor told me to read a book on how to test for demand with new ideas, The Mom Test. (My notes here). The biggest lesson it taught me that I thought might be relevant for you to think about: "If they're not already looking for a solution, they're not going to try or buy mine."

Hope you don't waste your time while uninformed like me :D

A different approach could be to first get in touch with, say, 2-5 people who are (semi-)professionally interested in such a discussion space, and optimize that for starters. Maybe that'd mean just scheduling recurrent calls, or having an email exchange. Once you have a strong core group it should be easy to expand more publicly.  

Besides that, I don't really have much to add to the answers (and a comment! ) thus far. 

As an aside: EA groups on FB generally seem like a coordination failure (due to FB probably being an addictive platform that optimizes for something like users' time spent). So for every active FB group it seems beneficial to create a non-FB group that could replace it.

(I know someone who created several online communities.)

I think the major issue is the external cost of the "institutional space" you take up: once you create the space (and tag/market it) it makes it hard for someone else to do the same. 

Because of this, it seems that if someone is taking up a new space, I guess moderate management quality, a minimum positive experience, and virtuously stepping down if needed, are key things they should provide. 

From the OP's post, my guess is that they easily pass any bar and should just do it (unless time/energy cost is a concern).