Going from 1 attack to 2 attacks (not using your bonus action) increases the chance to land sneak attack by 24%.
Bonus action or not is still irrelevant there. The benefit in Nick is in keeping that bonus action available.
Going 2 to 3 attacks (not using your bonus action) increases the chance to land stunning strike by 9.6%.
That depends on the assumptions being made in the accuracy to hit in the first place. Easier targets and it's a whatever moment, harder targets the more attacks the better. But it's a 33.6% improvement over 1 attack using the accuracy assumptions you seem to be using.
Here, this was a quick spreadsheet that should show what I mean.
| | | | | |
Attack bonus: | 6 | | | | |
| | | | | |
AC | 1 Attack | 2 Attacks | 3 Attacks | 4 Attacks | 5 Attacks |
10 | 85.0% | 97.8% | 99.7% | 99.9% | 100.0% |
11 | 80.0% | 96.0% | 99.2% | 99.8% | 100.0% |
12 | 75.0% | 93.8% | 98.4% | 99.6% | 99.9% |
13 | 70.0% | 91.0% | 97.3% | 99.2% | 99.8% |
14 | 65.0% | 87.8% | 95.7% | 98.5% | 99.5% |
15 | 60.0% | 84.0% | 93.6% | 97.4% | 99.0% |
16 | 55.0% | 79.8% | 90.9% | 95.9% | 98.2% |
17 | 50.0% | 75.0% | 87.5% | 93.8% | 96.9% |
18 | 45.0% | 69.8% | 83.4% | 90.8% | 95.0% |
19 | 40.0% | 64.0% | 78.4% | 87.0% | 92.2% |
20 | 35.0% | 57.7% | 72.5% | 82.1% | 88.4% |
21 | 30.0% | 51.0% | 65.7% | 76.0% | 83.2% |
22 | 25.0% | 43.8% | 57.8% | 68.4% | 76.3% |
23 | 20.0% | 36.0% | 48.8% | 59.0% | 67.2% |
24 | 15.0% | 27.7% | 38.6% | 47.8% | 55.6% |
25 | 10.0% | 19.0% | 27.1% | 34.4% | 41.0% |
I bolded where your numbers came up on the sheet. There's negligible increase in the 4th or 5th attack using Flurry if the goal is Stun but there's definitely a significant increase in that 3rd attack. The Flurry would be more about damage or other potential effects.
Stunned is action denial, concentration breaking, ends any effect that ends if incapacitated, and an autofail on STR and DEX saving throws. Sneak attack is typically more damage. Monks can add more damage and increase the likelihood of stunning high AC targets with that bonus action.
The rogue is keeping the bonus action using NIck pretty much for Cunning Action (dash, disengage, hide). The monk is keeping the bonus action using Nick for an Unarmed Attack, Flurry of Blows (even more attacks), Patient Defense (Disengage, or Disengage + Dodge, or later Disengage + Dodge + Temporary Hit Points), or Step of the Wind (Dash, or Disengage + Dash, or later Disengage + Dash + take a friend).
Monks have a lot of options for that Bonus Action. That 3rd attack is still a significant increase in landing Stunning Attack while keeping that Bonus Action.
Here is what it looks like later with a +5 ability score and +4 proficiency bonus....
12 | 90.0% | 99.0% | 99.9% | 100.0% | 100.0% | 100.0% |
13 | 85.0% | 97.8% | 99.7% | 99.9% | 100.0% | 100.0% |
14 | 80.0% | 96.0% | 99.2% | 99.8% | 100.0% | 100.0% |
15 | 75.0% | 93.8% | 98.4% | 99.6% | 99.9% | 100.0% |
16 | 70.0% | 91.0% | 97.3% | 99.2% | 99.8% | 99.9% |
17 | 65.0% | 87.8% | 95.7% | 98.5% | 99.5% | 99.8% |
18 | 60.0% | 84.0% | 93.6% | 97.4% | 99.0% | 99.6% |
19 | 55.0% | 79.8% | 90.9% | 95.9% | 98.2% | 99.2% |
20 | 50.0% | 75.0% | 87.5% | 93.8% | 96.9% | 98.4% |
21 | 45.0% | 69.8% | 83.4% | 90.8% | 95.0% | 97.2% |
22 | 40.0% | 64.0% | 78.4% | 87.0% | 92.2% | 95.3% |
23 | 35.0% | 57.7% | 72.5% | 82.1% | 88.4% | 92.5% |
24 | 30.0% | 51.0% | 65.7% | 76.0% | 83.2% | 88.2% |
25 | 25.0% | 43.8% | 57.8% | 68.4% | 76.3% | 82.2% |
26 | 20.0% | 36.0% | 48.8% | 59.0% | 67.2% | 73.8% |
27 | 15.0% | 27.7% | 38.6% | 47.8% | 55.6% | 62.3% |
28 | 10.0% | 19.0% | 27.1% | 34.4% | 41.0% | 46.9% |
29 | 5.0% | 9.8% | 14.3% | 18.5% | 22.6% | 26.5% |
That's also ignoring the 1's and 20's rules. And later.....
12 | 100.0% | 100.0% | 100.0% | 100.0% | 100.0% | 100.0% |
13 | 95.0% | 99.8% | 100.0% | 100.0% | 100.0% | 100.0% |
14 | 90.0% | 99.0% | 99.9% | 100.0% | 100.0% | 100.0% |
15 | 85.0% | 97.8% | 99.7% | 99.9% | 100.0% | 100.0% |
16 | 80.0% | 96.0% | 99.2% | 99.8% | 100.0% | 100.0% |
17 | 75.0% | 93.8% | 98.4% | 99.6% | 99.9% | 100.0% |
18 | 70.0% | 91.0% | 97.3% | 99.2% | 99.8% | 99.9% |
19 | 65.0% | 87.8% | 95.7% | 98.5% | 99.5% | 99.8% |
20 | 60.0% | 84.0% | 93.6% | 97.4% | 99.0% | 99.6% |
21 | 55.0% | 79.8% | 90.9% | 95.9% | 98.2% | 99.2% |
22 | 50.0% | 75.0% | 87.5% | 93.8% | 96.9% | 98.4% |
23 | 45.0% | 69.8% | 83.4% | 90.8% | 95.0% | 97.2% |
24 | 40.0% | 64.0% | 78.4% | 87.0% | 92.2% | 95.3% |
25 | 35.0% | 57.7% | 72.5% | 82.1% | 88.4% | 92.5% |
26 | 30.0% | 51.0% | 65.7% | 76.0% | 83.2% | 88.2% |
27 | 25.0% | 43.8% | 57.8% | 68.4% | 76.3% | 82.2% |
28 | 20.0% | 36.0% | 48.8% | 59.0% | 67.2% | 73.8% |
29 | 15.0% | 27.7% | 38.6% | 47.8% | 55.6% | 62.3% |
30 | 10.0% | 19.0% | 27.1% | 34.4% | 41.0% | 46.9% |
Again, ignoring 1's and 20's. Here's with the monk capstone ability score bonuses a multiclass gives up....
14 | 100.0% | 100.0% | 100.0% | 100.0% | 100.0% | 100.0% |
15 | 95.0% | 99.8% | 100.0% | 100.0% | 100.0% | 100.0% |
16 | 90.0% | 99.0% | 99.9% | 100.0% | 100.0% | 100.0% |
17 | 85.0% | 97.8% | 99.7% | 99.9% | 100.0% | 100.0% |
18 | 80.0% | 96.0% | 99.2% | 99.8% | 100.0% | 100.0% |
19 | 75.0% | 93.8% | 98.4% | 99.6% | 99.9% | 100.0% |
20 | 70.0% | 91.0% | 97.3% | 99.2% | 99.8% | 99.9% |
21 | 65.0% | 87.8% | 95.7% | 98.5% | 99.5% | 99.8% |
22 | 60.0% | 84.0% | 93.6% | 97.4% | 99.0% | 99.6% |
23 | 55.0% | 79.8% | 90.9% | 95.9% | 98.2% | 99.2% |
24 | 50.0% | 75.0% | 87.5% | 93.8% | 96.9% | 98.4% |
25 | 45.0% | 69.8% | 83.4% | 90.8% | 95.0% | 97.2% |
26 | 40.0% | 64.0% | 78.4% | 87.0% | 92.2% | 95.3% |
27 | 35.0% | 57.7% | 72.5% | 82.1% | 88.4% | 92.5% |
28 | 30.0% | 51.0% | 65.7% | 76.0% | 83.2% | 88.2% |
29 | 25.0% | 43.8% | 57.8% | 68.4% | 76.3% | 82.2% |
30 | 20.0% | 36.0% | 48.8% | 59.0% | 67.2% | 73.8% |
Not all targets have the same AC, and as the AC goes up the relevance of more attacks goes up with it when we're talking about important attacks like Sneak Attack or Stunning Strike.
