I'm heading on a short trip to South Korea soon. I will be based in Changwon. I wanted to know how feasible it would be to do a short trip to Wolchulsan National Park. Geographically it's not that far. Is there a direct bus between the two places? Would it be possible to do it in a daytrip? Am I instead better off looking at national parks closer to Changwon, perhaps Gayasan or Jirisan? I would be relying on public transport. Not on a strict budget so short distance taxis aren't too much of an issue if necessary.
Any info is much appreciated,
Thanks,
Owen

To get to Wolchulsan from Changwon, inevitably you have to make a transfer at Gwangju.
From Changwon Bus Terminal, buses to Gwangju will depart at 09:20, 11:10, 13:00, 14:50, 16:50, 19:10 respectively, KRW 13900 one-way, 3-hour ride.
Then you catch your connection to Yeong-am Bus Terminal. It is almost certain that the same bus serves multiple destinations other that Yeong-am, even though other stops are mostly 'after' Yeong-am in case of outbound from Gwangju. Still you need to pay extra attention to the destinations of the bus. It'll be roughly a 2-hour ride, KRW 6900 one-way.
And then at Yeong-am Terminal, you choose which sector of Wolchulsan you wish to visit and catch your second connection accordingly. You won't have any English signs or support on this leg, so you need to be able to discern destinations written in Korean to catch the right bus.
Nevertheless, one of the most popular access point is Cheonhwangsa Temple which is in the same sector as the National Park Management Office, and the sector's parking lot/bus stop is within 15 minutes' driving from Yeong-am Bus Terminal, which means taking a taxi is reasonable if your bus from Gwangju arrives at an odd timing to connect to the infrequent bus to the sector.