Got the RetroTink 2x today, it works great.
Now about using RT2x with the VGA adapter and OSSC;
The RT2x outputs exactly 828 total pixels, so you have to sample the right amount of pixels in generic 4:3 modes for shimmer free image.
That means that when the OSSC multiplies the resolution you have to set H.Samplerate in multiplies of 858.
For correct H.Samplerate in the OSSC, multiply 858 by 1.5 for x3, by 2 for 4x, etc..
The VGA adapter will only map correctly with Limited range HDMI signal which the RT2x outputs.
Also, the adapter has severe chroma clipping/distortion with very bright colors (especially cyan), test with SMPTE Color Bars 100% and high output console like 1Chip SNES.
This chroma distortion does not happen when connecting the RT2x directly to a display.