Definitely do not get breastfeeding advice from an AI. That is just going to regurgitate all the rubbish that is all over the internet.
It sounds to me like your LO is uncomfortable with their cold and keeps popping off the breast because they like to breathe through their noses while feeding, so if their nose is all stuffed up, they can't breathe and feed at the same time and they are so little they don't realise/understand why, so they just get frustrated. All of my children have done it. The best tip actually is to squirt your own milk directly from your breast up their nose, it helps loosen the snot and unblock it and then they can breathe a bit easier and stop getting cross while feeding. If you have no luck with this you can buy little saline drops/spray at any chemist for babies, which does exactly the same thing.
Your supply is most likely 100% fine. Once you've got through the first 4-6 weeks it's very robust, and it is unlikely to suddenly go anywhere.
Do be aware taking decongestants like lemsip can reduce your supply - but the effect should be temporary so if you've done this accidentally, don't panic, just keep feeding, drink plenty of water and it should come back. Just to say to avoid them if you weren't already aware.
Best thing for a baby with any kind of minor illness is breastmilk and lots of cuddles - they can digest it so quickly that it helps keep them hydrated. IME you are in for a disturbed night but it should pass in a couple of days. If you have family around to take care of you and take the baby at any point so you can sleep that would be helpful. Don't panic, it will be fine!
Oh and PS I could never express anything more than about half an ounce. I breastfed in total for over 10 years when you count up all three children. So I completely agree that being able to express or not tells you nothing about your supply. These videos are really helpful: