AB is definitely the difference maker. It’s kind of key to the whole shebang, because your most efficient, baseline level of healing revolves around churning out as many 0 charge ABs as possible. Regeneration is bugged: it is amplified by the AB debuff but doesn’t consume it like the tooltip indicates, which slightly harms your HPS and makes the rotation less fluid.
But just in general you’ll want to AB>Regeneration/Mass Regeneration>AB, then depending on your HPS needs and Clearcasting procs you will fork here. If healing needs are low you can just wait for the debuff to be ~2.4 seconds from falling off and start casting another AB, which will result in a 0 charge AB cast. If you hit a proc you cast AM into another 0 charge AB. If healing needs are moderate you can cast a second AB before either waiting for the debuff to fall or utilizing a Clearcasting proc for AM. Whenever you need to reapply Beacon you will want to do that during the highest AB debuff count that you are reaching. If things get really bananas you can cast a third AB and then either cast a fourth if Clearcasting procs, or dump it with a Regeneration/Mass Regeneration, regardless of Beacon needs (from what I can tell they are better HPS and HPM than Arcane Missiles).
I’m not sure if peak HPS is spam-casting AB or if you still want to dump the charges due to the healing amp, but as you can see your general goal is to cast as many of the lowest charge AB casts as possible, while maintaining Beacon on the targets you wish to heal, and while otherwise dumping your charges as efficiently as possible. I haven’t done BFD yet, but so far as I can tell this is how the build is set up to function, and AB is absolutely crucial to make it work as a proper healer. It is a very proactive healing style and you need to stay ahead of the incoming damage to succeed. Especially without AP and PoM our ability to recover from heinous where we have nothing set up is limited.