JD
I share your pain.
Microsoft have been appalling handling this issue. In true IT fashion they assume it is all your fault and you must have done something to **** it up. With so many reporting the same issue it is annoying this issue is not even acknowledged.
The only way I have been able to resolve it (after trying all the things you did) was to purchase a AUD$30 USB-C to HDMI cable which I now connect from my SF7 to my widescreen monitor (QWHD).
Resolved the resolution issue. TICK
Quality is not as good a Display Port connection
Downside is that I now have two cables connected to my SP7 (dock and USB-C). The whole point of a dock - even one as connectivity challenged as the Surface Dock - is to avoid this sort of set up.
Will it be fixed? Perhaps. However my recommendation to any one interested in a SP7 is "do not buy".
I should say my Surface Go doesn't suffer this issue.
RCC