I assume a fighter with STR 20 wielding a Greatsword +1 (with a generous GM maybe a +2).
Her boni are: +4 (proficiency bonus) + 5 (STR ability bonus) + 1 (item bonus): +10
So she needs to roll a 7 or higher (7 + 10 = 17 --> hit): 70% chance
She gets buffed by Bless: 75 - 90% chance
She gets advantage (by a spell, or something else): 91% chance
She gets Bless + advantage: 93,8 - 97,8% chance
so I think at level 10, it's reasonable to assume to have a hit chance of around 85% or more, especially when you think about Bless being a Level 1 spell (and don't ignore the bonus to saving throws! it's such a great spell!)
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 ?
