Oh Well P. can't win them all, hope you are all well. Cheers Will.

Can I use cassettes from mega system 2 to other consoles?

Hiya on the website where is the composite board please as I can't find it? Was going to check it has the capacitor 220 micro farads  and resistor inline 75 or 100 ohms on it as this may contribute to the effect your seeing. Good video keep them coming 😊

Just a thought: the TV says SCART so are you using a composite to SCART adapter? The pattern of repeated pixels makes me wonder if the TV has incorrectly interpreted the composite signal as S-Video which is a thing that I've seen happen. I have two old LCD TVs that both have the same behaviour, there's two SCART sockets: AV-1 is RGB/S-Video and AV-2 is Composite. Composite in S-Video causes effectively every other pixel to be wrong in a regular pattern.
The other thing that springs to mind is it was a common thing on mega drives to remove the RF unit entirely if you planned to use RGB exclusively as the very presence of the RF unit caused regular interference on the RGB. I don't know if it is relevant for a master system 2, but it's a possibility.