Most mummy will recommend S1. I didn’t use that before so I can’t comment on it. I’m using baby express BeMine and switched to BeFree (main caretaker so I need my hands). No problem with the yield.
Advise is… if you don’t mind latching, latch as frequent as possible, drink lots of water/soup. Don’t look at the amount while pumping (you can use your phone or play with baby) and most importantly, don’t stress. Not everyone is blessed with good amount from the start and it can get very stressful when you feel that you don’t have enough to feed. Just pump regularly, eat well and do mix feeding if needed while you build up your supply. I only transited to full BM at 1.5m. Fighting!! ✌🏻