The frost worm is Huge and 40 ft long, 5 feet wide vs this thing at 25 ft long, 10 ft wide. So why not start at Huge and advance to Gargantuan or even Colossal?
Yeah, improved grab + swallow whole. Those swallowing mechanics are pretty standard for 2e. Do we want to replace the acid damage with save or mold spores once swallowed?