If it's about enjoyment then surely if they reduce the amount it won't change your enjoyment?
Reduce the time the missions have to be completed in I say to less than 1 hour, will signifcantly reduce stacking and the ability to wait/mode swicth for new missions.
but that WILL reduce the enjoyment as without stacking you'll have significantly less NPCs trying to interdict you, meaning far less excitement (if any at all), meaning a far less interesting or enjoyable experience. the whole point of this is it's tense because of the amount of NPCs trying to murder you while you cross 300-400ly of space for a pretty hefty profit. make it only one mission at a time and flying 400ly for 5mil doesn't have any tension as a) the stakes are significantly lower (i.e. very low for the vast majority of people who can get these missions) and the risk even lower with very few NPCs, you've just sucked the fun out of it. nerfing this is killing it