Please forgive me if these calcs are misleading, but here are some I made:
252 Atk Darmanitan-Galar Ice Punch vs. 0 HP / 4 Def Gyarados: 157-186 (47.4 - 56.1%) -- 82.4% chance to 2HKO
252 Atk Darmanitan-Galar Icicle Crash vs. 0 HP / 4 Def Gyarados: 178-210 (53.7 - 63.4%) -- guaranteed 2HKO
252 Atk Darmanitan-Galar Ice Punch vs. 0 HP / 0 Def Haxorus: 284-336 (96.9 - 114.6%) -- 81.3% chance to OHKO
252 Atk Darmanitan-Galar Icicle Crash vs. 0 HP / 0 Def Haxorus: 320-380 (109.2 - 129.6%) -- guaranteed OHKO
252 Atk Darmanitan-Galar Ice Punch vs. 0 HP / 0 Def Stonjourner: 102-120 (29.9 - 35.1%) -- 16.1% chance to 3HKO
252 Atk Darmanitan-Galar Icicle Crash vs. 0 HP / 0 Def Stonjourner: 114-135 (33.4 - 39.5%) -- guaranteed 3HKO
Even with Defense/HP EVs, Crash deals slightly more than Punch and has a good 30% flinch chance.
Those calcs are accurate to what I've run, so I'd say they are fine. The one small caveat I'd make is to account for accuracy in them, since that will be a factor in whether or not Icicle Crash connects.
So for Gyarados, Icicle Crash would have an 81% chance to 2HKO, with it missing at least once 19% of the time. Compared to Ice Punch, these two are so close that it honestly doesn't matter which one you run.
For Haxorus, Icicle Crash would get that OHKO 90% of the time due to accuracy, but that's STILL better than the OHKO chance for Ice Punch. You'd want Icicle Crash here.
Stonjourner would have a 72.9% chance to get hit 3 times in a row by Icicle Crash - much better odds to pick up the KO in 3 turns than Ice Punch.
So far for me, the only times I've found that you'd want Ice Punch over Icicle Crash would be for if Ice Punch can guarantee you the OHKO (or 2HKO if Icicle Crash also gets the 2HKO). The only times I've found so far where that can happen are specially defensive Mandibuzz and Hawlucha:
252 Atk Darmanitan-Galar Ice Punch vs. 252 HP / 4 Def Mandibuzz: 248-294 (58.4 - 69.3%) -- guaranteed 2HKO
252 Atk Darmanitan-Galar Icicle Crash vs. 252 HP / 4 Def Mandibuzz: 282-332 (66.5 - 78.3%) -- guaranteed 2HKO (81% to hit twice)
and
252 Atk Darmanitan-Galar Ice Punch vs. 0 HP / 4 Def Hawlucha: 326-386 (109.7 - 129.9%) -- guaranteed OHKO
252 Atk Darmanitan-Galar Icicle Crash vs. 0 HP / 4 Def Hawlucha: 372-438 (125.2 - 147.4%) -- guaranteed OHKO (90% to hit once)
Looking at it now, I'd say that you're right, Icicle Crash would be the overall better option to run due to it's power, I underestimated how much of a difference 10 BP would make. Even if Ice Punch is guaranteed to snag KO's that Icicle Crash could POSSIBLY miss, Icicle Crash is still better to go with.
Edited a misspelling because I noticed it immediately after posting and it annoyed me.