Hi there and well done on getting this far. If you would prefer to BF rather than express I'd definitely suggest you get someone to check over your latch. Also keeping putting your LO to the breast now and again, as they may get more efficient over time and return there fully at some point.
As for expressing, it's hard work but definitely worth it. To answer your questions, yes I always stuck to a rigid time scale for expressing, I religiously do it 5 times a day for 15-30 minutes a time, at set times (give or take an hour if it clashed with LO feeding, etc). They do advise that you express as often as your LO feeds but I couldn't sustain that at home when my LO was on 12 feeds a day! Make a decision on what suits you, stick with it and your supply will increase over time. We did have to supplement with 1 formula feed in the early days but this soon dwindled down over a few weeks. If you think you've a dodgy pump, do get it checked out and I believe that most hospitals will let you borrow one if needed.
Also wanted to add, when you express, you should let all the milk come from the breast and continue to express for a few extra minutes as this will stimulate further supply. It does take time to increase though and you'll have to be patient.
One tip, have tummy time with you and your LO both naked, for a few hours if you can do it, as this is supposed to help increase supply. Also make sure you eat and drink plenty and try to be relaxed. Stress really does affect the amount you get, could that be why you've been getting a bit less this morning?
Any questions please don't hesitate to ask. I've been exclusively expressing for 13 weeks now and just love meeting others going through the same thing. Wishing you lots of luck xx