Unable to post in the convention forum, since I am not registered for the convention, so posting in the main forum.
1. After the DLS visit there seems to be no other railfanning activity.
So atleast while returning to SBC, do take 66531, but get down at BYPL and then you can take the Metro from BYPL to SBC. This can give the Metro experience too and a chance to see the city a bit. It wont be crowded since you are getting in at the starting point.
2....
more... Or after the DLS visit, if you finish early or if 66531 is late, you can take a BMTC bus from K.R.Pura market side bus stand near the DLS, to BYPL Metro station. All the buses to Majestic / Market will go via BYPL Metro station. Then from there, Metro ride to SBC.
.
Just a suggestion, it's up to you to consider.