NCH

I use their accounts package, and a few of their other packages, I am satisfied.

http://www.nchsoftware.com/

Microsoft.

Slack's desktop client is really just Electron I believe

That's the direction consumer desktop apps should move. Cheaper to develop, maintain, update, etc.

that's arguable

Slack is just a web app wrapped up on the desktop

You don't need the desktop clients as far as I'm aware.

Slack is aimed at businesses