First, just get an earlier bus. The SBB website will show you the shortest connection they think you can make. You can click on the advanced search function and put in longer connection. When I put in 10 minute minimum transfer time, it gives a bus departing Unteraegeri Zentrum at 5:25 and arriving at Zug in front of the train station at 5:46 which is plenty of time to make a 5:58 train. The 5:25 is the first bus of the day. You have 12 minutes to walk 300 feet from the bus to the train platform, to a Swiss, that's an eternity.
Second, what happened to the 6:31 departure from Zug we were talking about on the other thread?
Third, if you want more time to make that transfer in Zurich to Chur, there are trains departing Zug at 6:03, 6:05, and 6:29 that will get you to Zurich with more transfer time there.
The Swiss timetables always give you the fastest possible connection because they assume you want to get to your destination ASAP. But you can adjust the connection time.