I can’t really say one way or another if Serbia should join the EU, I don’t know if that is in Serbia’s long term best interests or not.
I will say that a split front in Serbian Political leadership at the moment is a huge mistake, one that may be too difficult to overcome to do much of anything constructive about Kosovo’s Unilateral Declaration of Independence, the last time one of those was used it was Rhodesia, and it failed...badly...
It's their choice - the EU or Russia.