For a large commercial plane on landing, does the L/D ratio increase, decrease, or not change much?
In the flare you are already in ground effect. Maybe we should compare the final approach with cruise.

The answer is simple: L/D is far worse in approach configuration. The wing buys the increased lifting capacity with a relatively higher drag increase. This is not all bad: If the L/D is too high, a precise landing becomes much harder. Also, some more drag allows you to run the engines above idle, so spooling up for a go-around takes less time. Generally, you want the L/D to be between 5 and 10 for landing. Too much, and the approach becomes too shallow for a precise selection of the touchdown point, and too little and you lose too much energy in the flare, so you decelerate too fast in that final phase where sink speed is reduced.

Compare that to the L/D of modern airliners in cruise, which is around 16 for designs like the Boeing 747 and above 20 for the Boeing 787 or the A350.

You can study this cheaply in gliders. Take one with powerful flaps, like the ASW 20 or the ASH 25. Try to land them with all flaps and spoilers fully extended: You will never get to a smooth touchdown, but drop on the wheel somewhere before the flare is complete. Then try to land them with flaps set for slow flight and no spoilers: You need a loooong runway to do so.

The lift contribution of the engines during the flare is very moderate. They do not run at full power, and even if they did, their combined thrust would be only maybe 30% of the aircraft weight. Since they don't point straight up, but only by about 10°, the vertical component would be only 5% even at full thrust. In reality, they run at a little above idle, and their lift contribution is only 2% at most.



Flap type effectiveness comparison (picture source). The L/D is only for maximum lift, not at 60% of it which would be more representative for an approach lift coefficient, but the trend to lower L/D with higher maximum lift coefficient should be obvious.