Singapore or Pyrgos are my bets. Colombo is too far west, even if that makes it closer to Rhomania and there is a good argument to leave it semi-autonomous due to it's closeness to India, while New Constantinople is too small and isolated, easy to pick off.
Meanwhile Pyrgos is in what will likely be the jewel of Rhomania in the East, the Heraklions, and is by OTL measures a very good location for a city, though subject to typhoons. The Heraklions are also primed to be the manpower base of Rhomania in the East once it is consolidated and the population starts to really grow.
Singapore is probably the perfect location, at the heart of Rhomania in the East and the Malay people who we know are likely to be the most loyal Rhomans in the East. It can also be a planned city with no existing ties to any one region which helps keep the others from feeling subservient to a former ally/rival. It is also sitting on a spot that will be absolutely critical to future shipping and trade from East Asia to Europe/India/Africa.
Location Singapore is better.
Pyrgos manpower base is totally dependent on local food source(that includes transportation to in and from farms, ports and inlands). The Spanish in OTL never resolved this until 19th century. The population under Spain was stagnant for more than 200 years since the population of 1500s would be roughly the same or near 1700s.
There is also potential for gold rush(copper and silver as well) in Heraklions especially the Mountain province, Benguet area to prop up population. Although gold was being mined there since ancient times, the Igorot's resistance and lack of interest of the Spanish to fully conquer them and apply heavy mining in the islands in otl, there wasn't really any large scale mining until the 20th century. Just to give an idea on how large the potential could be, in OTL 20th century the islands produced more gold than California, but less than Alaska.
The advantage of the Romans over the Spanish would be infrastructure building(building roads to the mountains). And abundance of soldiers comfortable with 1000-3000m of mountain elevation.
However, it would be upto b444 since the Romans can totally ignore the gold, silver, copper at the Igorot lands just like the Spanish. But the mines are there, and the Igorots will trade or have been trading gold, silver and copper at the lowlands(in this case already controlled by the Romans who might have a curiosity for gold and silver).