I feel that the mares retain their reproductive health better if they are bred every year. Typically, the mare will tell you when she needs a year off.
THIS! Every mare is different. I have a mare that is 17 and does wonderful getting pregnant, staying pregnant, foaling, and recooperating. My other mare really seems miserable toward the last trimester so she gets a year off here and there. She has a longer back and gets huge which contributes to her discomfort. It is very important, however, that you support their physical and mental well being with fantasic nutrition, turnout, vet and farrier, care, turnout buddies, exercise. etc…
Don’t push it…if you are on your 5th try getting her pregnant this year and she doesn’t take maybe she wants/needs a year off! Barring no infections or weird stuff!