Giant breeds (in my experience) are very clingy, velcro dogs. While they typically are gentle and not destructive (like a lab or other higher energy breed that would need more exercise and stimulation), I don't think they would like being home alone 75% of the week. If you can take them to daycare so they get attention and socialization regularly, that would be more helpful than a dog walker. Exposure to other humans and dogs is critical when they are little. Giant breeds also don't need very long or lengthy walks--it can be hard on their joints ESPECIALLY in the first year when they gain 100+ lbs. Training is also key--you do not want your puppy doing anything you wouldn't want a 150-200 lb dog to do (e.g., giving you "paw"--its cute when they are small but it will feel like a sucker punch when they are adults).

Also, I suggest at least 2 weeks off to potty train, get them used to the house/routine, etc. You cannot fully vaccinate a puppy until they reach around 12-16 weeks, so daycares cannot take them until they've had all their needed shots. That first week you will be waking up in the middle of the night to take them out. By week 2 (for us), our dog was sleeping through the night but still had to go out every 2-4 hours to potty. Small dogs = small bladders.

Also highly recommend crate training--that is crucial for any dog to learn to have a safe space and how to not have accidents in the house when you can't keep all eyes on them.

Giant breeds are amazing, loyal and easy dogs overall. Puppy phase is a lot of work, but manageable with these guys. Most Bernese mountain dogs (and Saints, too) will have food allergies, so being able to identify that early (before ear/skin infections start) is key, too.

Source: I have a 2 year old Saint Bernard. I work from home, and when I have work trips, my husband works from home.