I just received my RG300 in the mail today. After flashing RetroFW 1.2 (the latest public release) to a microSD (a SanDisk Class 10 64GB that I bought off Newegg) and popping it back in, I get a blank white screen upon powering it up. I'm aware of the ComicBook firmware, but I don't like its interface. I much prefer the GMenu2X type.
By the way, the date 2019.6.3 is printed near the internal microSD, which feels relevant.