It is best not to boil with the sugar as tends to split the milk.
The addition of besan is entirely a personal choice. I like the nuttiness it gives the dessert and the mixed flavours of Mysore Pak and payasam together.
You can use condensed milk to add thickness to the payasam. If so use after the milk has come to a boil.
you can add ghee roasted nuts to flavour however I did not want to load up on the ghee and hence did not use it.