Gulls are hard, and that is fine
I spent two hours on the harbour wall with a field guide open at the gull pages and came away with more questions than ticks. Large gulls take four years to reach adult plumage and every year looks a little different.
What actually helped: ignoring the brown birds at first and learning the adults properly, looking at leg colour and bill shape before plumage, and accepting that "large gull sp." is a perfectly honest entry in a notebook.
The black-headed gulls, at least, were easy. Winter plumage now, just a dark smudge behind the eye.