Shared vs not shared XP

Can you please give me some more info how this setting works?
I understood that if I use a full group (6 members) there is no difference between setting shared or not, since the xp that is given to the “bench” character is “bonus”.

I understood like that. Lets consider a figh that give 600xp.
Shared XP 6 member party:
I will have 100xp to every group member and to every bench character.

Shared XP 4 member party:
I will have 100xp to every group member and to every bench character.

Not-Shared XP 6 member party:
I will have 100xp to every group member

Not-Shared XP 4 member party:
I will have 150xp to every group member

Will I be able to reach lv20 with a 6 member group?

You understood it right.
Most people finish on level 17-19. But yes you can get to 20, question is, how easily/ how soon. DLCs content helps a lot. Reading guides helps a lot too.

*This is from my personal experince.

To do it without DLCs, you will need to pass all skill checks, do all optional side content to get to 20 lvl few minutes before boss of 8th chapter. Note depending on your choices your run can be won by defeating boss of 7th chapter. Chapter 8th is “secret” chapter.

If you decide to go solo with shared exp turned off you can get lvl 20 with your MC around start of chapter 4, I think. Party of 5 people should be able to gain enough exp to get 20 even on non-completeonist run (with DLCs finished). 4 (and lower) man paty is garanteed to hit 20 lvl.

You can also farm random encounters in chapter 6 and later, to do it sooner.
Changing Shared EXP during prolog also helps a little. You tun it off at the moment you can, then turn it on before companion joins, then turn it off again, rinse and repeat until before Valerie joins (running into building on fire) after that keep it on.