Can Zakat be paid at a rate of 2.5% monthly from the salary instead of paying it in a lump sum at the end of the year due to working part-time, and are periodic charitable donations counted as part of Zakat?

Firstly: Zakat on wealth is not obligatory unless it reaches the nisab (threshold) and a full hawl (lunar year) passes over it. It is permissible to pay Zakat in advance before its due time, according to the majority of scholars.

Secondly: It is permissible to pay Zakat in monthly installments if this is done as an advance payment before its due time. However, once it becomes due, it is not permissible to delay it and pay it in installments.

Thirdly: If you have given amounts as voluntary charity and did not intend them as Zakat, they are not counted as Zakat, as the intention of fulfilling the obligation is a condition. If you did intend them as Zakat, then you must have placed them in their designated legitimate categories of expenditure.