GSM works fine with RGB too, but you won’t get anything progressive out of the console unless it’s Ypbpr or RGsB (RGB with sync on green).
2x mode will really emphasise any jaggies in the image but otherwise I’d agree, if you can get progressive out of the console it’s the way to go. On a 4k TV, adding CRT monitor like scanlines I think gives the image a super pleasing look and just takes the edge off those jaggies enough. Make sure your TVs scaling engine is good enough to render them evenly though else they will look terrible.
and while OSSC is often berated for it’s interlace handling, my current preferred method of playing 480i PS2 games is actually with an OSSC connected to my multisync CRT VGA monitor. This very much looks like interlace on a consumer CRT TV (I don’t have room for a CRT television in my setup too).