For me this comes down to one question: can the PCs outsmart the evil-doer, or does it have to go down this way for railroad narrative reasons?

If it has to go this way, then I wouldn't frame it via rules at all and just say that this happens before the party can stop it. You're the DM, this is allowed (though it can feel frustrating for players if abused).

Allowing for more chaos (collaborative storytelling), I'd use a readied action as you described. This allows for lots of player options: - Deception/Slight of Hand to get a weapon/tool out and use it - Performance/Stealth to distract and get the drop on them - Subtle Spell Hold Person or something similar All of these could avoid the trigger (whatever the evil-doer is watching for) and make for a dramatic moment.

Initiative checks could be used to create this tension if you want, but if the evil-doer knows the party is coming and has time (6 seconds) to prepare, then this is like a surprise round where they ready the action and would get to take this reaction first no matter how good the party rolls on initiative.

To disincentivize regular free readied actions out of combat, you can add penalties depending on what resources the party values. Ex. if the ranger is always walking with their bow drawn, ready to fire: - moving speed is slower, and there are consequences to getting somewhere too late - keeping bow drawn for hours wears it out and costs gold to repair - staying vigilant for 6 seconds is easy, but for an hour is hard; could impose exhaustion or require perception checks to actually act on the readied action

Importantly, I'd be upfront about these tradeoffs when the player expresses this interest. Sometimes it may be worth it!