London Bridge – Gatwick Airport Commute

Hi all!
Approx three days a week I commute from London Bridge – Gatwick airport. It has to be London Bridge so I’m left with thameslink only I think. I have a 16-25 railcard. I’m confused whether the cheapest option is just for me to buy a peak day return every day from trainline and use my rail card as it’s above the minimum for a railcard to be used in peak hours, or if there is a cheaper alternative? Spoken to someone at the ticket office who seemed to have no clue. Would rather find the cheapest possible way to do this as it’ll add up over time! Thanks for any help. Also have an Oyster card with the railcard discount on if that’s useful to know.