I threw almost a dozen attempts at Byzantium in 1.19. I already have the Basileus achievement, instead I wanted to go for Mare Nostrum. Some of the most unlucky starts really. One time Venice beat the Ottomans to the punch with an early rival and DoW, it was not fatal, but the debt and manpower loss crippled me for the meager strips of land I gained. Another time I was able to ally Albania after the Ottomans declared into them, with Hungary helping, but the AI failed to group armies and got stomped. This other time PLC declined a CTA because they were at war with Teutons, losing, and had rebels. After that, I had an unusual time where Mehemet II died really early, replaced with some meek mediocre stat ruler that had the "Careful" trait, and curiously the Ottomans did not DoW anyone until I finally gave up on the speed 5 strat (1480+) and DoW'd into them with tech advantage, only to find out the Ottomans had both 'The Last Jousting Tournament', and a Morale advisor.
Then, when I was on the verge of calling it a day... I had a lucky break. Ottomans DoW into Albania, Hungary defended them, I was able to join, 99% warscore, and bam, all of my cores back. Not only that but I've had some incredible rulers who've lived good long lives. Even when I managed to lose my dynasty (heir and ruler both died within a year of each other), I still somehow managed to RNG into a 4/6/5 ruler with Russian dynasty. Though, I'd almost argue AI Spain got even luckier than I, with Burgundy Inheritance, Iberian Wedding, Portugal and Naples both integrated.
Even before this game I had an Austria into HRE run go very smoothly. PUs on Hungary, Bohemia, Commonwealth, Great Britain, Spain. This was also not without a few restarts, but that's Ironman for you. It can be generous and cruel.