The web app works fine. The desktop app is Electron-based but works fine. Yes, it is all centralised on their servers.
In which sense do you fear it will be invasive? Other than Slack seeing your messages on Slack, I don’t see what else you could be worrying about. They will always have that kind of access regardless of what you do locally since it’s not E2EE, so really this question just feels like paranoia.
Don’t say things on Slack that you don’t want Slack to know about. That should be all, really.
If you don’t want Slack to see any of your messages, don’t use it.
Proprietary Android apps are incredibly bloated nowadays. Like 400MB for a random single purpose app.
I dont trust them a single bit.
All android apps can communicate with each other via IPC, if both apps agree to it. This risk accumulates the more proprietary garbage you have on your phone.