Write a real Mac client.
Hire a Mac user to help.
Russell Finn commented
A native Mac client (instead of an embedded web view) would no doubt address a large number of other issues being reported here (e.g. non-standard user interface, command vs. option keys, spelling checks, needlessly firing up the discrete GPU).
We know Microsoft has native Mac developers working on Office 2016 for Mac; time to ask them for help, or send Teams over to them...