I love standardbreds and think they are amazing and versatile horses. That said, I don't think crossing standardbreds with other breeds works out well. More often than not they look like train wrecks. The sad fact is that standardbreds are an under appreciated breed and as a result have very little market value. To the point where, several nurse mare contracts, which require the mare is returned to the company in foal, will not accept a stallion as a father/stud. Mostly because they see the crossbred foals as having no value.
The traits you describe, willing, calm, athletic etc are not traits unique to your mare, that is the standardbred breed. Which is why if you really like the traits of this mare when you are looking for a eventing horse. I would suggest going to sales and buying a young prospect that has the conformation to support eventing. I would not breed I would buy a horse already on the ground.
Out of curiosity who is your mare out of?