This is odd... I have a royal marriage with Sweden, who have a 67 year old ruler and no heir. Their only other ally is Nassau, and they have no other royal marriages. Yet it states that a Swedish noble will ascend to the throne on monarch death. So you can't get PU's through royal marriage as the Netherlands then? I have an orangist in power
afaik you can only contest successions as the dutch republic. This means you can only attack a nation in a succession war, and enforce a union over the nation that fell in union.
you can't spread dynasty, but you can marry heirless nations to contest that they get dynasty spread from someone else, which then leads to a succession war.
Note that if the target heirless nation is NOT in the right tier/timeframe for this, the biggest monarchy RM partner spreads dynasty to heirless nation, NOT you.
So, marrying someone as dutch republic won't always lead to possible succession wars.
If it can due to right timeframe/tier, then make sure your military rating is highest of all rivals/marriage partners and dynasty partners of both heirless nation and the nation that gets the heirless nation in union. If you are highest, you usally can contest the succession aggressively in war. If you are weak, someone else might get that honor.
I can't comment on the Dutch Republic mechanics because I haven't looked at them properly.
Even as a monarchy, though, you might get that result. What are your respective Prestige scores like?
Prestige has nothing to do with dynasty spreading. The biggest effective development (a province with 50 autonomy and 10 dev only counts for effective 5) marriage partner spreads dynasty on death of heirless nation.