I finished the development and posted a video in my VK-community and Youtube-channel about creating another converter for charging high-voltage caps, this time powered by just one lithium battery. Its feature (which has led to both advantages and disadvantages) is the use of cheap and easy-available components, in particular, a PWM controller and a ready-made transformer from AliExpress.

Work on the project was difficult and took a long time - the first version of the converter, despite a lot of effort, refused to function normally. I also made a movie about this, which can be found in the description of the video. As they say, a negative result is also a result, perhaps it will be useful to someone...