I’m sure you are pretty confused by now.
But your main bike is your Domane and you can use that to get an idea of the gearing that might work for the 1200.
What is the lowest gear you use with Domane? For example, 34/28.
What is the highest gear you use with the Domane? examppe 50/12.
Since your 1200 is going to be heavier and probably have worse bearings, you probably need lower gearing on both ends. How you get there will be the suggestions you get if you can’t change the chainrings on the current crank and the freewheel/cassette (maybe rear derailleur for max cog and capacity). You do need to figure out if you have a cassette or freewheel.
Index shifting?
John