Unheated garage safe for seed sowing?

Currently in 6A/6B as a first timer with cut flowers. Purchased some seeds from Johnny's and started mapping out when to start seeds for Spring.

I have read about starting seeds indoors and also about winter sowing. However, we have limited space for starting seedlings inside the house and do not have a basement. We have a two story detached garage that is not heated. (See photo) On the second story of the garage there are large windows and plenty of space to start the seedlings. I like that the seeds would have access to sunlight but I am worried it may be too cold. Could I potentially amend winter sowing jugs and keep them up there? Or potentially a small greenhouse tent?

What would be my best location/ option for starting?

I am also fine with winter sowing if the flowers allow. It has been a particularly snowy and cold winter here this year.

Current seed list: Eucalyptus (silver plate) Snapdragon Statice Dahlia Strawflowers Sunflowers Aster Celosia Amaranth Zinnias Basil Cosmos Feverfew