Are the grandparents going to be as committed to toilet training, playing, socialising, general training, as you would be if you were there? What about the other day?
You have to put the work in, from the first day, if you want a well-behaved, trained, obedient, well-socialised dog in 18-24 months time. You have to be patient, consistent, prepared to stand outside in all weathers repeating 'wee wee in the garden, good girl' in encouraging tones for possibly 20-30 mins every hour or two. Consistent with bite inhibition training, every time. Doing 10-15 mins of sit, down, stay every few hours but also making it fun for the pup. I'm not sure that having 3 or more people in that role (unless they're very experienced dog/puppy owners) is going to be a great idea
I work from home and have mobility issues so I'm here 90% of the time. I don't leave them at all for several weeks, then start with half an hour, and build it up from there, but the maximum I will leave the dogs, even as adults, is 4-5 hours a day, and I live in the Algarve and they have access to the garden when we're out
Puppies are hard, hard work. They are cute for 5 minutes, then little bitey, jumpy, pee and poo terrors for up to 18 months depending on the breed. I've had dogs all my life (parents had a dog before us kids) and I've raised lots of pups, and taken on adult rescues, and every time I forget just how hard puppies are (and I recently had 3 within 6 years, so no long 12-year spells between pups)
Has the breeder not discussed all this with you?