I’m encountering same thing.
I can’t connect to the board, so not sure how I could burn a new boot loader…
I’m thinking that there is USB issue with Mojave. I’ve updated USB drivers (as per your documents) to the 2.4.2 version (latest on FDDI site)
No port shows up now so can’t connect to Seeeduino…
Other ideas? (I’ve tried this on 2 Macs, same symptoms)