Hello Hassan,
Do you mean a product of 2kg? That is a big chunk of chocolate, what dimensions will this be? I've no experience with something that large.
Anything that is rigid enough and can withstand the heat from the plastic, can act as a pattern. I've used wood, plastic, aluminium, cast resin, epoxy coated styrofoam, even plaster for thermoforming.
Obviously a CNC router and thermoforming machine are a professional solution to make this type of mould. But you're looking at quite a large investment in money (guestimate: several thousand dollars) and time (learning how to operate this machinery). I don't have a CNC router (yet!): at the moment I let somebody else do the CNC routing for any shapes I can't make myself.
PET thickness will depend on size, shape and complexity of your design. As you stretch the warm plastic over your pattern, it will become thinner in some places. For some shapes 1mm is ok, for others I use 2mm. E.g for large flat shapes I prefer 2mm, as it is more rigid than 1mm and there is less chance of cooling spots on the chocolate. But thicker plastic will need higher vacuum for good reproduction of small details and corners on the pattern.
My vacuum forming table is giving me good results for what I do, I make moulds for small shapes with simple geometry (largest so far was 20x20x5cm). First upgrade will be a better vacuum pump so I can use thicker PETG.
With thermoforming it is as with making chocolate ;-) There is no foolproof recipe: sometimes things don't work and you don't know why. Then you need some feeling and experience to balance all parameters for good results.
Best regards, Bart