Otherwise, keeping your reserve diplomats is more important, and you should only break (and then immediately remake) royal marriages when you get to 6 diplomats.As a nation grows, its army and navy grow with it. Thus, if you can regain stability very fast - one point in the time it takes you to generate two diplomats - then it is better to break the royal marriage as soon as you get to 1 diplomat.
But you can still break a royal marriage with one. One twist is induced by the fact that breaking a royal marriage takes two diplomats, if you have them, not just one. But other than that, you can get relations all the way up to +200 if you have the time. Often you will need to pay money to bribe the other country through the "gap" in relations, from -5 to +25, where royal marriages don't work. Generally, high relations are not so important relative to technological goals that you should channel your monthly income into stability if you cannot raise stability a point per year or so without any monthly income, then you probably should not be using this tactic. For a particular target country, assuming you can make royal marriages, keep breaking them and then remaking them as fast as you get diplomats and/or recover stability.