The foamboard you have is the same as we do, it’s true that it’s heavier, that’s why you can direct swap 3mm for 5mm to save back the weight. The paper does come off,
@CarolineTyler has a great process for taking the paper off.
The problems of weight are much less of an issue in the larger planes, I have both a Sportster and Scout that are made out the heavy board, I just added a little length in the nose and went one size bigger on the motors. More power is always good. Minis can be a problem and you can’t make master series designs out of our board very easily. Minis can be done fine in 3mm.
I agree it’s stupid we can‘t get the stuff like in America, but we managed fine before here was an EU distribution network. I don’t expect this problem to be fixed anytime soon so am planning to build designs that adapt well and work with what we have. Australia has similar issues and there are lots of foamboard fliers there too.