The Sundar Arpita relationship has been veryyyyyy slowly building up for a few months now. I think due to the exit of MV and the fast resolution of the revenge track, they've moved up the Sundar and Arpita relationship track to keep things busy until they get the future storylines planned and set up which is why it feels so rushed.
This show has done a super rushed and cheap wedding track before for Nishant when she was still living with the T's and her wedding with AKT was still a secret, so it's not out of the question budget-wise IMO.
Side romance stories are usually used to further the main character's own relationship and this feels like the case here - we already see how Imlie trying to get Arpita to close her eyes and see someone results in Imlie seeing Aryan when she closes her eyes.
I'm guessing we'll see more of BM and Imlie going head to head, Aryan and Imlie tension and relationship development as they try to help Arpita move on and then potentially the closing of the BM+BD chapter with their PF.