Hi Traveler000, and welcome to MySwissAlps!
>> I will arrive in St Moritz via Bernina Express bus then train on 30/08/17 around 16:30. Is a bus ride through Val Bragaglia and back, on the same day, worth it (3.5hrs)?
Well, that would definitely be a very long day, but in the end it's a personal choice. It is certainly a beautiful trip, especially going down the Maloja Pass.
If you arrive in St Moritz at 16:30, you will have missed the 16:15 bus to Soglio, and the next one departs at 17:15, getting you to Soglio at 18:45.
>> We plan to stop only at Soglio for 1/2 hr for photos.
If you arrive in Soglio at 18:45, the next departure would be at 19:27. Reservations for the 19.27 bus from Soglio to Promontogno are compulsory, and must be made at least 2 hours beforehand. I doubt it would be because of expected crowds, but more likely that if they have no reservations, they don't operate the service (just a guess on my part).
>> Is this drive beautiful enough for a 3.5 hour journey or are we better off just exploring St Moritz?
It is a beautiful journey, and Soglio is a lovely village with superb views, but it is a long trip for so late in the day. In the end though, it's a personal choice. St Moritz is a very attractive town.
>> Should we stop at other villages that would be considered prettier?
Many of the villages in this area are picturesque. None compare with Soglio for historical integrity, ambience and stunning views.
Sils Maria is a pretty village, and very close to St Moritz (about 20 minutes by bus).
>> Could we travel a shorter distance and experience the beauty of the valley just the same?
In my opinion, you would need to go at least as far as Promontogno. If you go that far, you may as well go the rest of the way to Soglio (12 minutes).
If you are interested, I have written a trip report about my trip to Soglio from St Moritz:
There are also some photos of the Maloja Pass attached to my post (Locarno to St. Moritz) of Nov 3, 2016 - 1:51 PM on the page below. On the same page are some photos of Sils Maria attached to my post of Nov 5, 2016 - 6:00 AM.