Oh dear, reading this I realise I have no idea how to use a rucksack, aren't you supposed to secure the hip belt around their waist?
I will look for some tutorial. I understand that some rucksack may distribute better the weight, and alleviate your shoulders and back, and that some are more durable, better quality.
I'll document a bit more and order one and make some tests at home. Thanks for all the advice!
Regarding the sleeping pad/mat am planning on getting one. What am not so sure about is if I need to get that mat that you install behind the tent. Is that very important or just to protect the tent?
You need to make sure that the hip belt sits on you pelvis, rather than just around the waist at its narrowest point; lower, in other words. Then a lot of the weight can be transferred directly to the pelvis, less on your shoulders! Makes a BIG difference, believe me!
As for mats, an insulated one is recommended for 4 seasons use. Cheapest, lightest and toughest are those closed cell foam mats, but being only a centimetre thick they are not the most comfortable. You can get insulated inflatable mats, but good light ones tend to be pricey. Keep an eye on the weight - some of the cheaper ones can be heavy! Uninsulated inflatables will not insulate you from the ground very well, although are fine for 3 season use.
Your sleeping mat goes between the sleeping bag and the groundsheet. Some people buy a tent 'footprint' that goes under the groundsheet, solely to protect the groundsheet from sharp stones etc. I have never bothered with one, waste of money for the sort of wild camping pitch that I choose (grassy), and extra weight to carry.