Do not count on a strait block! Their fort in Edirne will nearly guarantee they can cross. They also tend to not skimp on their own navy, so even controlling the strait is a hefty investment.
Hungary is the best ally. Poland has numbers, but Hungary starts witb strong traditions and a general, and their regency means they aren't burning manpower on the offensive at first. They will frequently accept offensive CTAs vs Venice, Serbia, etc if offering land.
Finally, you kinda have to get lucky and have ottos cripple themselves vs QQ or maybe mamluks in long wars. They don't fall behind in mil tech, are #1 disc early, and unlimited manpower. You could play perfectly and still lose.