
I would definately wait till she is at least 2 years old, look how mature she is then and re-evalute if you should wait longer.
By then you will also have an idea if she suffers from false pregnancies.
Your dogs breeder is right, Leonbergers can be sensitive to Anasthetics, I have had two of those myself.
On top of health issues/benefits, are you aware Leo's coat can be ruined by spaying and can be very difficult to look after?
Just one pointer to consider if two times a year few weeks season is easier to manage than a woolly mammoth all year round :) (I have had one of those too).
Pyometra is a risk but I don't think it is that common in a young healthy bitches and if caught early, can be treated.
I have had girls with little lumpy teats after seasons so I would just keep an eye to make sure the teats go back to normal.
If she is having a phantom pregnancy, her hormones are not back to normal till few months after her season.
Talking about phantoms, if my girl was suffering from bad phantoms, I would also spay when younger...
Phantoms can really mess them up for months. Season+phantom, we are looking at 3 months each cycle they are not "normal", three months normality and then the next season is due! In this case I would spay when they are about 2.5-3 years old as it is not nice for the dog or the owner to live with.
I have decided to keep all my future girls entire till they are little older and then spay to get rid the risk of having an old girl that gets pyometra.
Obviously 10 year old Leo in an emergancy operation would be very risky compared to a routine spay at 6 years old.