If I have a Weapon, that deals elemental damage (with for example a flaming sword or Elemental Weapon spell) of my draconic sorcerers chosen element, does that qualify for Elemental Affinity when I use a weapon cantrip like true strike?

This is, unfortunately, not entirely clear. It comes down to the fact that Elemental Affinity says it applies when you "cast a spell that deals damage of that type", and the question of whether the spell True Strike actually does damage or merely enables a weapon to do damage is still a matter of some debate.

This has implications for a wide variety of interactions like the ones you've mentioned. People on both sides of this argument are very, very sure that they're very obviously right and the other side is very obviously wrong, but there hasn't been any official clarification on it from WotC as far as I'm aware.

In practice, until there's a clearer official answer, it's a matter for each DM to adjudicate themselves.