Yeah, but Byzantium in 1399 (as it was in EU3) is one thing; by 1444 (as in EU4) the challenge is quite a bit steeper. You're down to three provinces plus one vassal, you're surrounded by the Ottomans who are extremely strong and they WILL be coming for you, your army is hopelessly much weaker than them, and getting alliances with more powerful nations is difficult. Fortunately your navy is still quite OK and the AI has blind and dumb spots.
Iirc in MEIOU the Byz start with 3 provinces (at most) as well. Not that this matters much; different game. But the ottomans also control the balkans, so the mod starts after the collapse of Serbia (maybe after they are even fully annexed; Bulgaria certainly has been annexed at the start).