I wanted to ask, is Dua Kumayl only accepted on Thurdays? If I recite it on a Monday, would I get the same reward/forgiveness/mercy that one would get on a Thursday?

Du’a Kumayl can be recited any day. Feel free to recite on a Monday and you will be rewarded for it. Yes, Thursday night is a special time, and all deeds are more blessed that night, but you can definitely recite it any other day.