Using PostMan, I could successfully POST to my CF testing endpoint and I saw all the right CORS-required headers, but I was getting the exact same browser errors in Chrome 75.
I solved my problem by implementing the solution from the out-of-date documentation here: CORS header proxy · Cloudflare Workers docs
This was similarly discussed on another CF Community post: Handling preflight requests - #8 by paul.b.hartzog
It appears that OPTIONS requests need to be specifically handled and that there are different responses required for variations even within OPTIONS requests…