Dynastic marriages and military alliances (Europa Universalis 4)

During the game, I got acquainted in some way with these definitions, but still questions remained unanswered. For example, he played for Lithuania, entered into a military and dynastic marriage with Denmark. For a millennium, I calmly developed the chosen nation and did not suspect that something could happen unplanned. But at one point, as I understand it, the king of Denmark died and there was a choice between my pretender to the throne and some other one. Of the allies, only one supported me, and against me there were six enemy nations. Taking into account such a case, I came to the conclusion that it is useful to make dynastic marriages only with co-religionists who are close in culture and with geographical neighbors. The same goes for military alliances. For it makes no sense to conclude a military alliance, for example, the Czech Republic and England. Even if it seems beneficial to someone, I think it would be unwise to fight your ally's opponents across half a continent. Another thing is if both allies are strong in the fleet, then their capabilities are much wider, since there is no need to “break through” through the borders of neighboring countries asking for the right of passage, going to a friend for help.
What are your thoughts on the above topic?