Vuman (Dex/ Wis) OH Monk, +2 Dex, Alert feat.
S: 10, D: 18, C: 14, I: 8, W 16, Ch: 8; Acrobatics +7, Perception +6, others, AC: 17, Initiative +9, HP 38, Move 40', Saves - Str +3, Dex +7 - Staff, shuriken (darts)
Vuman (Str/Con) Champ Fighter, GWM feat, +2 Str.
S: 18, D: 12, C: 16, I: 8, W: 13, Ch: 8; Athletics +7, Perception +4, others. AC: 18 (full plate), Initiative +1, HP 49, Move 30', Saves -Str +7, Con +6
Ok, so rather than involve actual dice rolls, let's work out some average results.
Assume the monk goes first. With 50% to hit, using two staff attacks and two unarmed strikes, they do 16.8 damage on average after factoring in possible crits (fighter is at 32.2). Averaging over number of hits and thus number of successful saves required, they have a 53% chance of being stunned.
Branch 0 (Fighter is stunned -- 53% chance): The monk goes again immediately, with advantage on all attacks. But they likely only have 2 ki left, so as you suggested, we'll suppose they use those to make two more stun attempts. (They might have more ki, as the fighter might have failed their first save, but they also might have fewer, with the fighter saving twice and the monk making a third stun attempt --- they might also have had fewer than two hits and been unable to make two attempts --- we'll say they have 2 and suppose that those alternatives are roughly a wash)
75% chance to hit now, with a total of 2d8+1d6+12 damage on the table --- 19.6 damage on average with the 9.75% chance to crit. On average, the fighter is at 12.6 in this branch, and has about a 54% chance of being re-stunned averaging over 0,1, or 2 attempts, depending on number of hits.
Branch 00 (Fighter is re-stunned -- 29% chance). The fighter goes down without a turn in this branch. The monk has a 29% win probability so far.
Branch 01 (Fighter is stunned the first round and saves the second -- 24% chance) In this scenario the fighter is at 12.6 HP, the monk is at full, and is out of ki (they might not be out of ki, but on average that likely means having had fewer stun chancesthat's probably a worse scenario for them than being out of ki).
Using the -5/+10 or not is pretty much a wash, so we'll assume they don't. 15.2 damage on average, factoring in the expanded crit range. The monk is at 22.8. On a crit, the fighter would get a bonus action attack, but we'll assume they don't take it even if they get it, and instead use second wind, to heal 10.5, back to 23.1. Action surge, for another 15.2, and the monk is on the ropes at 7.6.
At branch 01, the monk is at 7.6 and out of ki, the fighter is at 23.1, and has used action surge and second wind. The monk loses in this branch, because they don't do enough damage this turn to finish off the fighter, and they go down the next round. The fighter is up to a 24% win chance.
Branch 1 (Fighter saves stun attempts in the first round -- 47% chance). The fighter is up, and does 30.4 damage using action surge, with the monk at 7.6. There's a 19% chance the fighter crits, and if they take a bonus action attack they stand a decent chance of finishing off the monk then and there, but let's suppose again that they use second wind, healing from 32.2 to 42.7.
The monk is back up, at 7.6, and two ki remaining. But the HP difference is so big that the monk really needs three turns to finish off the fighter before they get a turn and down the monk. So basically, the monk needs to spread out their ki, making one stun attempt and hoping it sticks so they can do it again the next round. The chances that this works twice in a row are about 9%. But if that happens the monk can pull it out. Otherwise it's the fighter's match. 9% of the 47% in this branch is about 4%. So we'll credit the monk with another 4%, with the other 43% going to the fighter, in addition to the 24% they already had.
END RESULT: Monk wins 33% of the time; Champion wins 67% of the time.
Obviously some things were simplified here, but I tried to account for the biggest swings. It could be the monk could shave off a few more percentage points of win chance if we accounted for all possible stun save patterns, including those where the monk burned through ki more slowly. But I'm doubtful they get to 40% let alone 50%.