Sounds like you’ve had some excellent advice so far. Babies who have had a “later” tongue tie division commonly have a bit of oral aversion for a few days afterwards. Try just to keep her close and breastfeeding as relaxed as possible. Lots of side-lying or laidback if you can - take the pressure off both of you and go back to making it an enjoyable time together.
After about 6 weeks or so your milk supply is less reliant on pregnancy/birth hormones and instead driven by demand (how often and how much milk is removed). It makes sense that if there have been any background issues with your baby removing milk, they’ll start to show up now.
I didn’t see any mention of how often your baby is feeding? Day and night? Both sides? Not block feeding?
It might be worth getting back in touch with your LC. They really shouldn’t mind or charge for just a follow up phone call.
A couple of suggestions...
1- Breast compressions - if she stops actively feeding, gently compress the area of breast above her nose (google will explain this better than me), to give her a bit of extra and entice her to continue suckling.
2- Give a small “top up” after most feeds. If you give her one big extra feed, you won’t be giving your breasts the regular stimulation they need and she’ll be receiving it at a much faster flow than your breasts.
3- If feeding, pumping and giving a top-up feels too much (especially with another child), only do it every other feed. Or look up “power-pumping” instead. Dropping 2 centiles, the smacking and the tongue tie all suggest that if you do nothing, your supply will likely start to fall further and she’ll be very frustrated. But it’s also not a crashing emergency - she’s healthy and gaining weight. So there’s a middle ground for you to find between increasing your supply and not driving yourself mad in the process.
4- I would use the pump/haakah when she’s on the second breast. Babies of her age most likely will get frustrated with the slow flow if you pump beforehand.
5- Save yourself the stress of giving top-ups by bottle. Most LCs can also provide you with a feeding tube - it’s taped onto your nipple and dipped into a bottle or syringe of expressed milk. Sounds and looks complicated the first time, but easy once you know how. The advantage is that she takes it simultaneously as she breastfeeds - so you’re stimulating and she’s getting a top-up, saves a ton of time.
Take care, it sounds as if you’ve got a lot on right now x