That is mind boggling. How on earth can MS have an efficient knowledge management with this setup?
How frustrating was it too use for you?
The most sensible way forward seems to me would be just set up a single company SharePoint with a doc store with a predefined data layout. No associated team space around that. Move communication 100% to slack. Link / pin the relevant SP folders to the slack channels.
Just for the dev team this would work as we never moved chat from slack to teams. But we need to
1. interact with the less technical parts of our org (product management, quality & regulatory, BI, V&V, Legal etc)
2. Interact with colleagues from other subsidiaries.
They all use teams, so conversations & especially meetings will happen on teams, people will still store docs and notes there just out of convenience or lazyness.
I do not know how to solve that conundrum.