That was my exact situation. 2018 R model ... engine gave out on the way home from work. I had tracked it a full weekend 6months prior with no issue. It was an odd experience, but warranty took care of everything.
Do you have the OEM Carbon Fiber wheels? If so, I just checked with my local Ford dealership and unfortunately, I was told there are no options available for the rear (315/30/19). Unless, I'm missing any other details, please excuse me.
I'm not sure if it's been already mentioned elsewhere on this post or forums, can you briefly tell us/me why the base model was discontinued starting in 2019MY. Thank you.