While thinking about this, realised that each card would need to subscribe to events independently anyway so leaning towards having a script for each card which contains a method for its effect, and a Start method which subscribes to whichever event triggers the effect. Is this reasonable?

Hey, thanks for such a detailed answer. Architecturally, would the hardcoded OnCardPlayed method (which is just the effect so can be invoked at any time and not just when the card is played) require a new C# script for every effect in the game, or would it be better to have one script that holds a bunch of methods for all the different effects?