S-Video to SCART adapters are all passive, so you’re just changing the connector and doing nothing actually convert the signal to RGBS or YPbPr. An S-Video to component converter (not adapter; needs to be a box with circuitry, like this thing) would work, but I can’t speak to their efficacy.
You can also try tracking down an RGB decoder. I have a Kramer FC-4044 that works quite well in decoding composite video and S-Video to YPbPr/RGsB component and RGBS. Unfortunately, these things are very hard to come by; good luck finding one.
I would say the best solutions would be either:
1. The RetroTINK 2X, which accepts composite, S-Video, and YPbPr component, and outputs HDMI. It’s a very basic scaler, compared to the OSSC, but it, importantly, has compatibility with composite and S-Video. You would need to either forego the OSSC, or get an HDMI to VGA converter (which would be a bit silly, going from analog to digital to analog to digital); or
2. N64RGB modification, either borti’s or Tim Worthington’s, so you can actually get RGBS out of the console, and apply some deblurring compensation; or
3. UltraHDMI modification, for the same reasons as the N64RGB, just with HDMI out instead of analogue RGBS.