The recommended discard is 9m.
This theme is about in desperation of winning a hand.
With only 8,300 points, a big hand is desirable but at this stage, prioritising an easier win is more important.
Discarding 9m confirms the hand shape around the manzu area and has the highest number of tile efficiency.
Upon drawing 2m and then discarding 5p/1s, the tile efficiency for manzu is 5m and 7m.
Upon drawing 5m and then discarding 5p/1s, the tile efficiency for manzu is 2m and 7m.
If 1m is discarded at the start instead, the hand shape for manzu will be worse off as compared to example 1 and 2. See below example 3 & 4 for illustration.
Discarding 5p first is not bad but there will be a lower tile efficiency during 1 shanten (1 tile away from tenpai)
Due to 5p being discarded, this hand will not able to pon 5p or 2s.
If 9m is discarded instead, this hand can pon 5p and 2s.
Some players might discard 1s to prioritise getting Ittsu. However, in this situation, it will be better to focus on getting double ton and complete two sets.
Translated from: http://www.maru-jan.com/content/nanikiru_god/nanikiru_god_24.html