Honestly, if I had to start from scratch in my own company, I would pick zulip. That is thread friendly for real, for proper async communication. Slack threads were bolted on as a feature, and it shows. I know that in 2 this is not a problem, but the way that - for example - notifications work for threads when you have 10s (or 100s) of threads is a pain. Finding back threads you were involved with is a huge pain.