We already discussed this one and it's by far not as bad as you think.
It's true every size needs a different approach. I wouldn't say one takes more time than the next.
How about using 4 sheets acrylic for a bigger painting? If you use paper there's no need to use the easel. Just place it on the rable (a piece of tape in the middle if each sheet to keep them at their place.
The acrylic or watercolour sheets will work it's what I also use. That cardboard sucks it falls apart. Nevertheless, make every stroke count. Look before you start.
By the way, I did gloss/varnish my paper and some have very thick layers of acrylic (not needed?). It feels in the end not different from canvas.
I never made paintings bigger than A4 or A3 except those on walls and that's a different way of working.
Layers can make a painting more realistic but the question is if it's needed and what your style is. There's no need to be the next copy-cat.
Did you try water colour? White is rarely used since the white comes from the white paper. You could give a light wash here and there which comes from the water you clean your brush with.
Right now I struggle with a canvas where parts of paint refuse to stick. I scraped most of the paint off.
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You're right about several things, although, on this card, if acrylic without water ruins it, I imagine watercolor wouldn't even let me get started.
Why not? You can use it faster. You might not be able to use the drip technique but my acrylic is wetter than my water colour.
If those boxes arrive try to paint on the inside and see if it the acrylic works and what it looks like. If there's a difference with the cardboard you bought.
I used to paint on it a lot. Also made toys out of boxes like a garage for cars, a safe with money and a kitchen. All part of the December 5th celebration: St Nicholaas (the surprise gift you make for someone else).
By the way, I do notice a huge difference if it comes to paint, it's thinner/wetter, strange substance, fewer pigment. Red is hard to get. I tried adding the red powder I bought for colouring food. I can't tell if it has any effect.