What is "item bonus"? I understood that +1 on weapons increase their damage, not attack rolls vs AC ?
But if we will gain 3 bonus points ( +2 proficiency, +1 STR ) at level 10 compared to current lvl 4, it looks like we will lose -3 on AC , or even more. I do not know what is average AC for level 4 but , for example, Spider Queen has AC 14 so I assume average AC for lvl 4 is below that 14 ... and that means at least increase of 3 AC and maybe even more to match those AC 17.
In other words, it appears it will be exactly same miss chance at lvl 10. Advantages and Bless are available even now ( where I use first always when possible, and do not have cleric for 2nd ).
Unless that "item bonus" means that some better items have also +1 to attack rolls ?
As grysqrl already said, yes, a weapon +X gives you +X bonus to your hit roll.
I just looked it up, there are 167 monsters with a CR of 3, which should be the brunt of monsters we will be fighting with lvl 10.
Of these, there are ~120 which have an AC of 14 or lower.
So even without buffs and Larian's tweaks, the above mentioned fighter has - without any buffs - a to-hit chance of 85% against the majority of level appropiate monsters.
Please remember, when you fight against a CR 10 monster (while your party also being level 10), this is meant that your whole party fights against ONE monster in that encounter. Not several monsters. It is meant like a sort of dungeon boss or something, maybe go CR 11 for a big boss or so.
When you fight against multiple monsters, e.g. goblins (they have a CR of 1/4), then the rule of thumb is to use a CR of party level divided by number of monsters.
And I am aware that the Challenge Rating System of monsters is nota perfect system. But it gives you good examples, around what level of difficulty an encounter CAN be balanced.
EDIT:
There are already +2 weapons in the game. So, although I personally don't approve, we will probably get +3 weapons pretty soon in the game.
