I think the idea was to make it possible to run domains with little or no bloodline.
You seem to want to require regency costs for Ruling holdings as a way of making those regents with large RP incomes use up that RP. The goal seeming to be to level the playing field.
I think requiring regency costs for Ruling holdings will actually have the opposite effect. It will make it comparatively more difficult for regents with small bloodlines to rule their domains, leaving them with less of a surplus with which to oppose the Contestations, Agitations, Espionage, or other actions of those regents with tons of RP.
Put another way, by requiring more RP to be spent on routine actions, you make RP a more scarce and precious resource. When you do that, those regents with more of that resource have an even greater advantage against those with less of it than they did before. Vice versa if you diminish the make RP less scarce by not requiring it to be used on routine actions.
If you want a game where Avanil, Boeruine, Mhoried, and Ghoere are truly dominant and can do what they wish against rulers of lesser bloodlines, then by all means retain the 2e system. The 3e BRCS, however, actually levels the playing field more.
In my view, RP use should not be a requirement to perform an action. It should merely modify the actions. And I don’t think GB should modify actions as easily as RP, otherwise GB is clearly the best commodity. In other words, if 1 RP gives you a +1 bonus to resolve an action, it should cost you 2GB or more to give you a +1 bonus to resolve that action. That way, RP is more effective at modifying actions than GB is.
Alternatively, it might make more sense to raise the base cost of every action, perhaps doubling the GB required for each domain action. That would reduce the incongruity of spending 6 times the GB required to perform a 1GB action and only getting a +5 bonus (when you’ve paid the base cost 6 times over). It would also make GB less available for boosting the success chance of an action, letting RP fill in its proper role in that place.