What's your torch setup? Oxy/Propane is not only adequate to the job, it is much preferred over O/A for brass brazing crowns or any other large joint where you want everything hot at once. I include lugs in that category, but there are many fine builders who braze lugs slowly, only flowing a fraction of the lug at a time. Even for that slow lug brazing method though, I'd still prefer propane

O/A is severely limited in how big a flame you can get, unless you use a bigger acetylene bottle than almost any hobbyist would use. The max witdrawal rate is a proportion of the bottle size, so small bottles can only make small flames. Unless you make a manifold with a number of bottles all flowing at the same time, which used to be done in industry, not sure if anyone does that anymore.

When I "graduated" from acetylene to propane for fork brazing, it vastly reduced the time it took, just because of how much more heat there was in the propane flame. Never could have gotten that big a flame with any of the O/A setups I had used previously.

I can see why a hobbyist might want to braze the steerer-cown separately, and the blades-crown later, but that's hugely inefficient. Once that big chunk of crown is at brazing temp for the steerer, you should braze the blades right then and there, sez I.