For me, "in advance" means 提前. "beforehand" means 事先.

"事先“ it means a period of time before the thing you do. Thing uaually say something you have done or something has happened. In many cases, we do not use it in the future tense. “提前” it means something you do or something happened before the time you schedule. In Chinese, we usually say" 事先你就应该把它做好." and " 这件事你应该提前做."

literally, 事先 means before the thing, 提前 means make it earlier. so 事先 can only be an adverb. e.g. 开会应事先通知。 提前 can also be a verb except for an adverb. shift to an earlier time e.g. 运动会提前了。/ 开会日期从六月五日提前到六月一日了。 when it is used as an adverb, it has no difference from 事先。 e.g. 开会应提前通知。