I think your measurements may be off. If you are 5'9" tall and you have been comfortable on a size 60 bike, there is no way your inseam is just 30 inches. A cycling inseam is measured from the floor to your public bone, and more likely your cycling inseam is 33 inches or 34 inches.
To measure your cycling inseam, you need a book that is about 18 inches long and two inches wide, a tape measure, and a friend. Pull the book up against your crotch until it is uncomfortable. Then push the book an inch higher. Have your friend measure from the top of the book to the floor. Take the measurement about seven times, and the medium measurement is your cycling inseam.
An easier way to see if a bike fits is simply stand over it. I am 5' 9 1/2 inches tall. When I stand over my size 60 and size 62 bikes, the top tube is pressing slightly against my crotch. But, not to a painful degree. When I ride this bikes, they fit me perfectly.
For myself, there are three sizes of bikes that work for three types of riding. Using traditional geometry, a size 60 fits me well for urban riding in heavy traffic where I want a higher head position to watch motorists closely.
For faster riding on road with little or no motor traffic, I would want a bit lower bar position, and would ride a size 57 or size 58. If I were forty years younger, and racing on a closed course, I would select a size 55 or 56, to get the bars ultra-low, and make it easier to ride in an aero position.
Go to two or three shops, and tell the staff EXACTLY how you plan to use your bike (urban commuting, touring, fast touring, racing) and get their fit suggestions. Which ever shop tries to put you on a bike less than a size 55 is staffed by idiots, and you should leave immediately.