From the pics, it's definitely a 58cm. A 54-55cm would offer more wiggle room. The reach may be too much if you say you're getting sore riding on the current flat bar. The seat could be adjusted forward. A rough estimate is to be seated, with hands on the hoods of road brake levers, looking down...the top of the horizontal portion of the handlebar should block the view of the front hub.
This can help you figure out what road stem length to get. These days, there are drop bars with less forward reach, and shallower drops. This may also help in dialing in your comfort.