What is the ruling on obtaining Bulgarian citizenship under the aforementioned circumstances, knowing its religious and worldly benefits and that there is no harm to the right of return?

There is no harm in obtaining Bulgarian citizenship out of necessity. When taking the oath, intend loyalty to the Bulgarian Muslims and striving for the welfare of the people. If it is possible not to swear an oath of allegiance, or to restrict it to what does not contradict the religion, then do so. Try to emigrate to a Muslim country if possible.