let the seedlings grow first winter [works in Iowa] ... water big plant sparingly

You can do either. If your seedlings decide to go dormant--most of mine do--then let them. Cut back the water or they'll probably start to rot.

8-month seedlings are large enough that they're likely to play games with dormancy whether you like it or not. :-) Growth will certainly slow, and you can cut back watering to a more reasonable level to account for that--they won't need the succulent levels of moisture they liked in summer, but also won't fall all the way back to the monthly or twice-monthly watering a dormant plant would want.

One of mine refused to go dormant last year so I simply kept it under lights and kept watering and lightly feeding it. This year, as of this point, all are at least in a light dormancy (two have leaves remaining, but no active growth).

Generally, I don't "force" dormancy, I let it happen. Reduced daylight generally sets it off, and temperatures on the windowsills is way over sixty.

Coming back to say about 2 weeks after I posted I found they’ve all stopped growing. After resting for about 3 months there are new leaves now.