As I recall, the amount of ink that a nib will hold depends on the nib. Some of them hold more than others. The type of ink makes a difference. The paper makes a difference. I do not do enough flourishing to recommend a perfect nib, ink , paper combination. Hopefully someone else on the forum will respond.
I am guessing that the speed does make a difference, but would like to hear from others. When you are going back and forth between shades and no shades, I don't think you can go as fast as the flourishing that is entirely non-shaded.
As far as planning ahead for starts, stops, and joins - IMHO that is something that comes with practice. As you get good at flourishing, you get better at knowing when to stop to reload the pen and also how to make a join if you run out of ink in the middle of a stroke. In my years of taking workshops and watching demos, I've seen a variety of techniques. Most of us try everything we see and settle in on the things that work.
To build consistency, it is helpful to do some of the drills with a pencil. That way you do not have to do any stopping and starting. You can see some variation in the line when you apply pressure - which indicates where the shades would be.