I think what you have is perfect. My roosts start at 30 inches as well and even my overly chunky wyondottes jump/fly up to it now problem. You have the "step up" at 18 inches which is perfect.
When I saw the window ledges I was going to ask if they were roosting on them, I suspected they were. Chickens like to roost as high as they can physically get.
I can't tell how far that is from the floor to the window ledges but as the birds get older and start to put on their adult weight they are likely not going to be able to get that high without some type of step up and ramp might do that as well. It will also depend on the breeds you have. Heavy breeds probably not, lighter breeds like leghorns, hamburgs will.