Buses to and from Downtown Disney generally do not start until just before Downtown Disney opens for the day (9 or 10 depending on the season).
You can't count on getting a bus to a resort from Downtown Disney for the first hour or so that it is open. That's because Disney assumes there won't be any resort guests who need to return to their resort so quickly. So just because a bus delivers guests from the Contemporary to DD (for example), doesn't mean it will be returning to the CR. Bus dispatch is probably going to send it elsewhere.
FWIW, Disney does not want guests parking at Downtown Disney to go to the parks. They used to have buses between DD and the parks, but too many people were parking there (probably to avoid the parking fee) and the stores at DD complained that their customers did not have a place to park. That's why those buses are no longer operating.
You can give your plan a try and see what you think. Personally, I think the time you spend doing it that way will far exceed the time you might spend in traffic leaving a park at the end of the day. The buses to the resorts are usually crowded, too, and then you willlikely have to wait quite a while for the DD bus. Most of the buses are serving the parks at closing time.
When we drive and stay to the closing, we really take our time leaving the park. Shop a little, sit a little, watch the people and then stroll out slowly. By then the lots have cleared some and we get back about the same time (or a bit later) without the stress of traffic congestion.
Buses run between DD and the resorts until at least 1 am (when PI closes) and often up to an hour later.