Unfortunately, IME, long before #2, the PC's have failed a stealth check to bypass some guard or other, that guard has immediately raised the alarm, and the PC's have now had to slaughter everyone in the manor just to get to #2 which is immediately a combat encounter. IOW, the heist is basically just one running combat encounter with a largely pointless amount of discussion beforehand.
This can be helped if you have some kind of system to allow a certain number of auto-successes on necessary rolls. The plan should obviously work to create a situation where a minimal number of things are left to chance and thus rolls.
Also this is absolutely an area D&D falls flat on its face on, because it has skill checks which are both A) binary pass/fail, and B) extremely random with no centering whatsoever, so there's basically like a 30% chance that your skill investment even interacts with your roll at all in any meaningful way (i.e. not so low it doesn't matter, nor so high it doesn't matter). If there was ever any argument against using D&D 5E for heists, it was the basic design of D&D 5E's skill system. Though "min 10" thing Rogues can do can make a huge difference if they're high enough level and have it in the right skill.
The other big reason not to use D&D is that D&D has absolutely no rules for knocking out or killing someone from surprise. I'm going to work on a version of the Execution Attack rules from Worlds Without Number if I run any future D&D heists to deal with that (it's OSR so should work well-ish).
EDIT - Here's the wording on Execution Attacks from WWN (pretty sure no-one will mind given it's in the free version of WWN and possibly also SWN):
"Execution Attacks
A target that is completely unaware of danger is vulnerable to a quick and bloody death, no matter how great their martial prowess or how thick their armor. An Execution Attack gives an assailant an opportunity to slay a foe with a single well-placed arrow or blade.
Setting up such an attack requires a full minute of preparation. Archers, gunmen, and other ranged attackers must spend it judging distance, wind, and details of aim, while melee assassins must use it to drift up to the target and position themselves in the exact right place for the attack. Melee assassins must use a weapon for an Execution Attack, unless they have such special training
as to make their unarmed attacks unusually lethal. If the target is spooked, the opportunity is lost.
Once the preparation is complete, the assailant may use a Main Action to attack. The target’s Armor Class is irrelevant, assuming the attacker is using a weapon that can hurt the target. A melee Execution Attack will always hit. A ranged Execution Attack requires a Dex/Shoot skill check against a difficulty of 6 for a point-blank shot, 8 for one at the weapon’s normal range, and 10 for a shot
at extreme range. A Warrior can use their Veteran’s Luck ability with this skill check, but it only allows a reroll on a failed check rather than forcing an automatic hit.
If the Execution Attack hits, the target must make an immediate Physical saving throw at a penalty equal to the attacker’s combat skill level. If they fail, they are Mortally Wounded on the spot, or knocked unconscious if the attacker was using a plausibly non-lethal weapon. If they succeed, the weapon still does its maximum damage."
So in D&D terms, you'd need 1 minute of setup, regardless of whether you were using melee or ranged, probably limit it to proficient weapons and normal (not long) range, maybe even 30ft because D&D likes limiting things to 30ft, even though I think it's dumb lol. Depending on circumstances you might need Stealth checks and so on.
After one minute, you use an Action to make an attack. If it's melee, it auto-hits, if ranged, roll against AC 10 (w/in 30ft,) AC 14 (w/in normal range), AC 14 with Disadvantage for long range, or something like that. If successful, target needs to make a Fort save vs. the PC's Prof bonus + damage stat + whatever the normal number is or be reduced to 0 HP (dead or unconscious at player discretion as per normal D&D rules). If succeeds at save, target takes maximized damage instead (obviously including SA if it's a Rogue doing this). Rogues should probably get something making the save even harder to make, too, but maybe maximized SA is enough to account for that.
