I used Strength Sap Jellicent a lot to check G-Darm and I feel like it's probably the best check for this broken mon. Jellicent resists both Icicle Crash and Flare Blitz and do not take much from Earthquake, it still get 2HKOed by CB Earthquake but I never saw them lock into it much.You're forgetting to take into account Gorrila Tactics, which is currently bugged on the calc.
+1 252 Atk Darmanitan-Galar Flare Blitz vs. 252 HP / 252+ Def Avalugg: 192-226 (48.7 - 57.3%) -- 94.1% chance to 2HKO
+1 252 Atk Choice Band Darmanitan-Galar Flare Blitz vs. 252 HP / 252+ Def Avalugg: 286-338 (72.5 - 85.7%) -- guaranteed 2HKO
(+1 represents GT)
As you can see, Avalugg is definitely not a check to Darm-Galar, since it gets easily 2HKOd by both of its most popular sets as of now. Sure the opponent has to get some predictions right in order to really take advantage of this, and Avalugg can OHKO back with Body Press, but Avalugg not only can't switch in without fear of getting Flare Blitzed, it also has to find time to recover against something else on the opponent's team unless you want your Avalugg to be potential death fodder.
Edit : I forgot, Marvel Scale Flame Orb boosted Milotic is a dumb check to G-Darm, but accessing Strength Sap and Water Absorb makes Jellicent way more reliable than Milotic imo.