Pumping regularly is better as it tells your body that milk is needed. If you leave it for longer than 3 hours you are risking to loose the supply. Plus pumping smaller amounts often amounts to more milk than if you pump or longer stretches.
Definitely do every 2 hours to start with and drink oat milk. It was a game changer for me! I had to do every 2 hours, and then my supply went up and now I do it every 3/4 hours!
Also be sure you’re using the right nipple flange. If it’s too big the pump wouldn’t work adequately and your supply would continue to reduce.
You don’t have an established supply yet. You need to signal that your baby needs more milk so every 2/3 hours for at least at least 15 minutes. Pump for 15/20 minutes even if your milk stops coming out.