I made some research and yes, It looks like with 12th gen PPW it's possible to make it working. By setting [pdoc] instead of [ebok] etc, etc.
However after all, since new features of PPW6 wasn't that important for me, I finally decided to buy PPW5
In my specific case it will be just more convenient, because now I still can use AZW3 (for my native language ebooks which KFX doesnt support). And add hyphens to them etc, etc. And use KFX only for english books.
Also I don't have to worry that something wont work suddenly. I just do everything as always. For example I can resize manga cover (made with KCC), by using only resize cover plugin. With one click. And Kindles shows it in proper size, even if its not cover embed into file.
My PPW5 came yesterday btw. I was little worried about its battery, since this Kindle probably spent around 2 years in shop. But it came with 50-60%
I already uploaded some books and manga and everything works great. Also I didn't expect such a big change from 10th gen PPW. It feels really different and better.
I was also wondering about bigger screen in the past (for reading manga), but now I know that 7" is max for me. 6"8 of PPW5 is already big enough for me and bigger device would be inconvenient to use in my case.
I'll follow which way Amazon goes in the future, if they fix their problems with sideloading and make it better, or even worse. So when it will be time to change to new device I'll decide if I stay with Kindle or change to Kobo or something else. However it's a matter of the future, since I expect that PPW5 should serve me for many years from now.
Thanks everyone for participating in this topic!