I have some of the same issues, but I'm not part of any community slacks. I run my own consulting company, and as such, I've been invited to most of my client's slacks. So I've currently got 37 Slack workspaces going. But most of the time, I'm only concerned with the specific project channel that I'm working on for a client, and I might only be active in 2-3 projects at a time. So I'd love, like OP, to be able to just have those 3 channels front and center.