If you analyse Mawashigeri, you can see that Mawashigeri consists of various different elements, which have to work smoothly in synchronism. However, you might have a certain difficulty with a specific element of Mawashigeri.
If you want to improve a certain element, my advice would always be to create a training method that isolates that movement. The extension of the leg for example, is obviously one of the elements. If you perform Mawashigeri as you do in basics, you have to not only mind the extension of your leg, but also your balance, axis rotation, hip movement, kamae, pivoting of supporting leg.. etcetera. All of these elements hinder you from focussing purely on the extension of your leg. For that reason, you can practice lying down on the floor as pictured. This allows you to isolate the extension of the leg and practice without worrying about any of the other things mentioned.
This method emphasises mainly two things. One is becoming mahanmi completely and the other is balance of Axis rotation. When you execute Mawashigeri for basics, you have to open up your hip completely, meaning that you should become mahanmi. This method allows you to focus on this and check at the same time since you have to put your kick straight down after completion. Heisoku dachi is used, so that you can prove that your axis rotation and balance is under your control. Usually in basics you step through or step back, which allows you to correct your balance by adding movement. If you use Heisoku dachi, you cannot hide any balance issues. Also, it helps you to focus on your Axis movement, because you have no intention to step through or go back, or in fact follow up with any movement that can hide your inabilities.