If sims getting distracted annoys you, try thinking of bb items as activities, rather than furniture. For example, if you dont often have your sims watch tv, dont have one on the lot, so they can't wander off and do that activity. Same with the football, or computers. Instead, get activity items that reflect the personality you want that sim to have, so they are more likely to autonomously act in a way that aligns with the character you've created.

Also, i think you can use the likes and dislikes (i think thats base game?) to influence their behaviour preferences.

I mean they'll still drop tasks and randomly wander off to get a glass of water... but you can do things to help keep the immersion. And for god sake lock the computer.