I'd appreciate any suggestion for 2 days visiting Mount Qingcheng + Leshan.

You can go to Qingchengshan on the 1st day and go back to Chengdu for stay overnight. On the 2nd day, go to the Buddha. There is no direct transport between Qingchengshan and the Buddha by the way.

At Chengdu Railway Station, three rail stations are scheduled to Qingchengshan, at 06:48, 16:20 and 19:16. It takes an hour. And at Chengdu Xinnanmen Bus Station, there are many buses to Xiaoba Tourist Bus Station from 07:30 to 19:30. After getting off, take a taxi to the Buddha. The way back is the same.