Yoga for Strength isn't about just doing poses to mindlessly get stronger. It is about directing your awareness and your effort so that you can apply strength effectively with less effort.
If you put your effort in the right place, you don't have to use so much of it.
For stronger yoga poses the first step is to create stability somewhere within your body.
The better you are at creating stability, the easier it is to get stronger with less effort.
If you are having problems developing strength it may be due to a lack of stability somewhere within your body.
Notice where you lack stability, so that you can work at creating it. Then you can get stronger with less effort.