If I were you I would plan on getting a dedicated set of winter wheels and tires. Get a square set of 19” or 20” wheels and a good set of winter tires. You will NEVER have the slightest problem getting around. You can probably find a 19” or 20” square setup on the G20 forum if you don’t have luck on the G60 forum. Probably 235 or 245 tires. You need to watch the sidewall aspect ratio and diameter. The diameter doesn’t necessarily have to be quite as big as with your 21” setup, but it needs to match front and rear, which, of course, a square setup will.
You can use this calculator to make comparisons of what you have to what you’re looking at.
https://tiresize.com/comparison/