Option #1 seems simplest. Option #3 discriminates against individuals who sell direct, without using a marketplace. A possible option #4 is: establish EU-wide

Option #1 seems simplest. Option #3 discriminates against individuals who sell direct, without using a marketplace. A possible option #4 is: establish EU-wide rules that can optionally be applied to cross-border transactions. I.e. a seller could choose to follow the EU-wide rules when selling into another country. This doesn't require countries to change their own internal rules.