I have a Cocoon Create/Duplicatir ic v2 printer and I have had problems with the first layer for years up until recently. I have always used the Kapton Tape with a mixture of wood glue and water but your glass bed should be better.

With the tape and glue it sticks very good if you have the bed level and the right height which I have only recently done.

https://hobbyking.com/en_us/kapton-h...0mm-x-33m.html

Until recently I could never get the height of the bed right so I recently printed knobs which I put lock nuts in.

The VERY loose wing nuts that came with the printer were useless to keep the bed at the right height,It is one of these but can not remember which one. I printed a couple first but did not fit a M3 lock nut.

https://www.thingiverse.com/search?q...15c936e52793cb

I am not the one to give advice on your comment "I have a question about how the first (very first level) of plastic should look. Should it be completely flat or should it also look like a roll?"As far as I can see it should be half way between the two you mentioned but quite flat on the top.

Now I have the print sticking to the bed the next thing is to make sure the bed is the correct height all the time. Someone said I should do two Skirts around the print before it starts printing the item. So I do four skirts just to make sure on all prints. What the skirts do is give me time to make very minor adjustments to my bed lock nuts.

As I have lock nuts I only have to do very minor adjustments to get the print right.With the glass bed you need advice on getting the print to stick and then my lock nuts and printing four Skirts allow me to get the height correct.

After years of trying I now can get the first layer to stick properly.

All the best with yours.