There’s no clean sync output on the PS2 so you will need a sync cleaner for that. PS2 only outputs sync on green at 480p.
On the Dreamcast I believe a modded VGA box will output 480i clean sync but I can’t say for sure.
I don’t know about the input side if Retro Gaming Cables can’t help, did you try Keene? For the output side you should really have 75ohm sync and you might also need voltage on the RGB up pin on the SCART. Try ArcadeForge’s UMSA – http://arcadeforge.net/UMSA:::15.html