The rule of thumb from Daina Chiba’s book says 3 pairs = break a pair. To decide which pair to break, you look at how good the broken pair is for your hand.

Discard 8p - good cards to draw are 679p. 69p gives a closed wait, 7p gives an open wait

Discard 4s - good cards to draw are 135s. 1s gives a closed wait, 3s completes the straight, 5s gives an open wait

Discard 6m - good cards to draw are 357m. 37m gives an open wait, 5m completes the straight.

46m is easiest to upgrade into a good thing so 6m is the best discard. 

I think so, at least!

i pick 6m without thinking time... wanna see other choices with reasons 🤔 (especially if red 5s are on)

I also like 6m. It's easier to upgrade compared to 224.

Pure efficiency:

2S, in all situations, the edge tiles (1-9, 2-8) are having significant disadvantage over the middle tiles (3-7) as their improvement tiles are numbered significantly less than their middle counterpart. Here, the 446M can be improved with [2M, 3M, 4M, 5M, 7M, 8M] (24 total) while 224S can be improved with [1S, 2S, 3S, 5S, 6S] (20 total), and which is better now better seen.