Does anybody know of or recommend a service for messaging use that has a good desktop linux program for small business use?
Probably paid for so we get some support with it.
While it doesn't have paid support,
Jitsi is a free & open source option that's been around for awhile:
A couple of other messaging programs for business use besides Slack:
Troop Messenger
Mattermost
Ryver
Tox
Jami
Also: Franz is a good tool for multiple private and business accounts managed at the same time.