I am not a quick learner. Never mind graphs and that, I just know I had to work hard at idling for several long sessions before I could do it. Ditto with freemounting, reversing, and even turns in each direction. It seems so long ago.
I bought my 28 with 110 mm cranks and was able to idle it with reasonable ease on a smooth gym floor the first time I rode it.
With 102s, I now find it is idleable as long as I show great care.
I have no doubt that with 150s, it would idle almost as easily as the 20.
A 29er is only marginally bigger than a 28 (3% bigger) but it is a bit heavier. So if I can do that on a 28 with 102s, then you can do that on a 29 with 125s, with practice.
I don’t want to get too heavily involved in the maths, but a simple comparison of leverage shows that if your 29er has 125mm cranks, that is in a sense equivalent to a 20 with 86 mm cranks. However, that simple comparison misses much of the story.
You don’t have a 29 to do tricks and stunts on, but to ride. The skills you need to be a good all round rider are freemount, ride straight, turn smooothly, turn sharply, stop under control, idle briefly, dismount neatly. Anything else is a bonus.
The reason that idling a 29 might seem difficult is that it begs you to ride it far and fast. When you are simply riding far and fast, you are learning/practising nothing except riding. It takes a certain amount of self discipline to devote a few minutes of each ride to practising idling or mounting or reversing.
The trick to idling a big uni is not to think of rocking backwards and forwards, but to think of moving in a controlled way from one position to another.
Position A is wheel forward, uni just starting to tip backwards, and position B is wheel backwards, uni just starting to tip forwards. You need to “ride” smoothly from one position to the other at low speed, rather than simply “pumping” like you do on a 20.
Although I blow my own trumpet a lot by writing up my adventures, there is abundant evidence from many witnesses in this forum that I am not a particularly talented rider, so if I can idle a 28, I’m 100% sure you can idle a 29. But it will take time and practice.
Have fun trying. It’s a skill that will improve your riding pleasure, not a duty, or an exam. A bit at a time. Try stopping under control then starting again. Get better at the idling. Always dismount of the back. Maybe shorten the cranks on your 20. Learn to idle the 20 one footed, and idling the 29 two footed will be easier.