Hi,
In European 90's, the SCART plug in RGB mode was:
-> 1V/75ohms composite video input with or without color subcarrier, sync/luma ratio 30/70%
-> 3x 1V/75ohms RGB 0-100%
-> 1 control pin for incrustation 1V / 75ohms
very often in RGB product, the composite video was PAL, even for the French market with SECAM color process.
In the analog TV set, the bandwidth was ~8MHz in RGB mode from the scart plug to the CRT. It was a great improvement. (vs ~3.5Mhz en PAL/SECAM).
It's an interesting project, don't give up !
Theo