don't worry too much and good luck

Your income alone is sufficient enough to support your parents. Some people suggest bringing tons of documentations/pictures for interview. All I prepared for my parents are the invitation letter, the forms, an employment letter from HR and a printout of online banking statement. And only invitation letter and forms were being requested during interview.
Seems to me that your parents' case is pretty strong. Just need to stress that they have been to developed countries in the past year, and you have been visiting them regularly. For the first-time visit, you might want to ask for a short 1-3 month stay.
Good luck.