We did roughly the reverse in 2023 and I don't remember it being long or painful. We maintained the Google driving times, but it was in early May. We chose different stops, but on the same route. We live near the coast, so beaches aren't a high priority.
Dunfanaghy(dep 9:30am) - Assaranca Falls - Glengesh - Glencolmankille Village (and lunch) - Slieve League - Donegal(arv 4:30pm).
One tip: We followed the signage to the Sliabh Liag Car Park and walked with most folks up the paved access road (about 20min powerwalk each way). When we got back, we asked the ranger why there were so many cars using the access road and he said "you just have to ask and we'll open the gate" which would have shortened the visit or lengthened the time at the top - that may depend on the time of year. Ice cream from the food truck was nice.