Not ADHD, but this is how I deal with Slack:

- Disable all sounds and vibrations of Slack on my phone (but keep the notifications)

- Disable notifications on my phone outside the work hours

- Mute all channels that I am strictly not interested in

- Disable all desktop notifications

- I don't have dual screen. Before I was a screen dedicated to slack, and it was actually an unnecessary distraction

With that, I only look at Slack whenever I'm not focused at something. If I find that a channel contains too much spam, I also mute it. If it's important, people will have to ping me.