26 Ac is generally the highest flat ac you can get with a very heavily geared character vs +14 to hit so the best change to hit vs the best armor is a hit on a 12 or better so 40% advantage makes that 80% disadvantage 20% a +1 would just be a 5% increase that is discounting exceptional stat bonuses from special items of course or ways to increase your stats over 20 but the standard stat cap is 20. So +5 and your proficency bonus +6 at level 20 and +3 weapons assuming there are +3 weapons anywhere in the game currently the game files go to +2 with rare weapons speaking of which the sword in the stone should be made into a rare weapon rather than just being a +1 long sword. Since its a named item. It would also be nice if you had more choice in what weapons you could craft with the sussar bark, So you could at least customize it to how you want to play rather than being niched into a specific weapon when crafting.
So anyways talking advantage vs disadvantage dnd doesnt exactly make alot of options for bonus to hit instead they give 2 chances at 40% for the best attack to hit the best armor rating. Which unless larian breaks that best possible circumstances thats fairly decient balance. Its still chance vs % to hit. Early access your best possible armor is 19 and the best possible to hit is +1 weapon +4 stat bonus + 3 proficiency bonus so your at a +8 which is 45% chance to hit with disadvantage its 22.5% miss chance vs 90% chance to hit against the best possible armor with advantage. Which is fairly balanced when you consider that alot of the enemies who are in lower numbers do have high health and multi attack with higher hit rates you need to use things to give you an advantage to make sure you are making those hits due to level limitations. The bullet for example on an average fight you need that advantage to kill it. With limited rounds to do so.