To smooth the transition between eras, especially between Antiquity and Exploration, I would also adjust the units, regarding continuity mode. In Exploration, for example, boats don't evolve but remain from the Ancient Era, and until the right technology is discovered, these boats cannot enter the ocean. I would also keep the remaining units from the Ancient Era class and create technologies to make them evolve. This would provide a greater sense of continuity and a bit more effort to ensure that units are always competitive.