ZDoom

I am a complete noob at scripting looking to start trying to do one thing. I had a look at the wikis but frankly couldn't find something obvious to me as "the thing you are looking for is here" so apologies for what likely seems a dumb question. The wiki was not immediately transparent to me. I want to make one (hopefully) simple global change to monsters; reduce the chance the alerted mob will choose pathing towards the player in preference to shooting if it has a clear shot. Not by turning aggression up to 11 like with -fast, per se, but maybe reduce the chance by a third. If I am understanding this correctly that is. I feel like it would be a very lightweight way of increasing enemy "AI" without impacting the way maps play TOO much or interfering with mods.

You'll most likely want to reduce the monster's MinMissileChance property. You can also set the MISSILEMORE and MISSILEEVENMORE flags to increase their chances to attack over longer distances.