You have nicely summarized the options and status. Yes, you are right. Unicode based stars or any other emojis will be easier to implement in expressions.
I too explored the Unicode based half star and I also could not insert it in the text expression.
Based on my understanding of unicode characters / emojis, I believe they have following limitations for our extensive use in business apps
Most of the emoji set appears to be lacking in emojis suited for business apps. They do have fruits, vehicle types, food and drink etc. but overall for business / industrial app use they have limited options.
We have lesser or no control to match their color formatting to match say app theme colors.
Most importantly, they are not available uniformly on all OS and devices.
Of course, we can continue to make limited use of unicode / emojis wherever feasible in apps.