Settlers of Catan is a great gateway game and the limited confrontation is part of the attraction for a lot of players. The card game can be surprising to new people with its aggressive nature and more defensive play that only appears in expansions to the original game.
Personally, I find that the game is based on attrition, with many cards acting as insurance against random disasters and interventions from the other player. The insurance mechanic does not offer deep strategy, with cards/events having single direct counters with success often based on dice rolls.
To make the game a more peaceful experience, try the following changes to the base rules.
1. Remove the Black Knight, Merchant, Bishop, Herb Woman, Spy, Arsonist and Brigands from the expansion cards. These cards form the bulk of the player driven confrontation and insurance options.
2. Remove Civil War, Plague and Conflict from the event cards. These events are negative in nature.
3. Ignore brigand rolls on the event die.
4. Trade advantage winner receives 2 resources from the bank instead of from the other player. This is a patch to allow trade advantage to be relevant whilst removing the ability to directly affect the other player.
5. The knight and windmill tokens are worth two victory points each. Many cards lose their purpose with the above removals. By increasing the worth of the knight and windmill tokens, buildings/knights maintain importance.
6. The progress event gives extra resources for each bathhouse, church and aqueduct instead of for the abbey, library and university. The bathhouse, church and aqueduct special abilities are no longer used, but the abbey, library and university special abilities are still relevant. This change is to re-balance the buildings.
7. Reduce the hand size to two. With less action cards, the number of building options increases and each hand has less dead cards. This change retains the relevancy of the redraw options and the power of increasing your hand size.
Optional. Lower the victory points required for victory to 10. Due to the reduction in variability of the game play, avoid the game dragging out.
With these changes, the game becomes a resource management race with the dice adding a factor of luck. There are no direct interactions that force the other player to lose resources or cards and there are no negative events. Overall the game will be simpler, but managing resources and competing on strength and commerce points still allow for strategic decision making.