Need help with rebuilding a Tony Kart BSS and BSD caliper

Hi all,
recently I bought a Tony Kart 401RR DD2 kart and I thought it would be a great idea to rebuild the brakes.
However, this adventure went sideways really quick.

I did rebuild brakes from BMW and Porsche models earlier with no hassle.
So I generally think I know what I am doing. But these brakes give me headaches.

I wanted to replace the sealings but I fail to install them properly.
The OTK break calipers have an additional clip and I have no idea how to install these properly.

I thought the clip doesnt wear anyway so I decided to not change it at all. I installed the sealings and tried to push in the pistons. I had to use a lot of force. I even used some tools as I couldnt slide them in with my hands. And yes I used break fluid and even some special break paste.

In the end I managed to install the pistons but it seems that they are stuck now. The pistons move only a little so I dont have any contact with the disc in the end, resulting in no braking.

Therefore I decided to rebuild them again. But what did I wrong?
What is the ideal way of doing so?

Do I install the clips first and then the sealings? In my experience the sealings are only laying on top of the clip instead of next to it.
I do not know how to proceed from here. Im happy with all hints and tips.

Also I watched some youtube videos (Power Republic) and it seems that there wasnt a clip there. Or he did not change it. At least he did not mention it in the video.

image (1)

Thanks guys!