Surely you want a very slight drop to allow for water to drain into the garden as
iona says it’s pooling on that concrete surface,
I would want enough space to get between shed and fence for any maintenance required
Also with a shed either side of what I assume is an access gate from the rear garden to a track which allows vehicle access, is there enough space for anything you want to get into the sheds (particularly with doors of one shed opening into your corridor)
Think you’d need to erect the big shed first due to space constraints.
Do you plan to have the bit between the two sheds covered like a sort of loggia?
I don’t regard myself as an expert on the subject but I did spend a lot of time looking at sheds this year so what I learned is fairly fresh in my mind