Property Booked in 2013 Agreement done in March 10 2015

Hi i gave an advance amount of 10 lakhs in Nov 2013 and got allotment of the villa with villa number from the builder I paid another 30 lakhs in may 2014 and some installments every few months however the Agreement for sale and construction agreement was franked on 10 Mar 2015 . i got Possession of the villa in November 2018 and moved into the villa after the interior was done in may 2019., i stay there now and my aadhar has this villa address. I have not yet registered the villa . now the value has gone up from 2.8 Cr to 6.5 cr. if i sell the villa without registration will i get the indexation benefit and considered as long term capital gains or short term capital gains.Do you suggest i Register the villa and then sell or can i sell without registering the villa as the registration cost i loose around 12 lakhs additional compared to transfer fee which i can pay to the builder to transfer the property to the buyer. If i sell after Registering will the indexation benefit be from the date of allotment or date of possession or date of agreement of sell likewise if i sell the property without registered from which date will the indexation be considered thanks