You are not alone. Chicken coop kits are rarely ideal it seems. The shed sounds like a good way to solve your space issue.
We have a coop from a kit as well & have had to make many modifications to make it work for us. Ultimately, our plan isn’t to revamp our older shed for a chicken coop before winter so we can insulate it for our cold winter weather. The manufacturer said 18-20 chickens. I have no idea how they came up with that number honestly unless they double or triple stacked them. The one roost was WAY too low thus they started sleeping in the nesting boxes. So I moved the lowest roost so that it was higher than the one the was the previous high roost and took the dividers out of the nesting area. Our coop had a nice covered attached run so I bought some free standing nesting boxes and made the run essentially an open air part of their coop and we added a bigger fenced run for their daytime enjoyment. During the day I leave the door between the coop and big run open so it cools it off should they choose to stay inside. However ours do get several hours of free range time by our seasonal creek area providing that the eagles aren’t hanging around.