It's unfortunate that Photos doesn't follow the industry standard here.
The only automated solution I know of would be the Search Replace Transfer plugin. You could use it to copy the rating (1, 2, 3, 4, 5) to keywords ("1", "2", ...). It wouldn't be the "1 star", "2 star", etc. that the conversion from iPhoto to Photos used, but it should be just as functional, since nothing in Photos attaches any special significance to the "1 star", etc. keywords.
Unfortunately, the plugin throws an error when you try to do that:
It would be a simple bug to fix -- you might send the author a note about it.
Thank you. Yeah, I never understood why Apple dropped star ratings. It's too bad there's an error with the plugin. If it were fixed, would it be possible to do "5 Star" and so on instead of just "5"?
I have photos set up to work with photos as "5 Star" and so on, set up that way since the conversion from iPhoto. I've already got thousands of photos in Photos. So, I'd like to continue with that if possible and not have any confusion with plain numbers as keywords, which I use sometimes.
I'm thinking of a temporary manual solution to do each time I have new exports from LR to go to Photos. I'll search LR by star ratings, then adding "5 Star" and so on keywords for any rated photos just before exporting them to jpegs and putting them into Photos. I'll just have to remember to do this.
Unless anyone else has a good workaround going?