This is really common at this age, I think, even in babies who are exclusively fed at the breast. It's the way they increase supply. I think that despite the supplement, it very well might BE a flow issues. All of mine did it.
What happens is baby starts to get impatient with slow flow. They're happy when you're letting down, but when the flow slows, they get ticked and want it to start again. Believe it or not, the popping on and off can elicit another letdown, and babies figure that out. (They'll also figure out all kinds of other weird stuff that will elicit letdown-- twiddling the other nipple, kicking, thrashing, and all kinds of other annoying habits!
). Over time, if you tolerate the habit, as baby keeps insisting on subsequent letdowns, your boobs will get the message to make more, and your supply will increase. It's a habit you'll see a lot during growth spurts, for instance. Newborns just go to sleep when the flow slows, but older babies are more active in their pursuit of MORE.
Now the situation is complicated if you're supplementing. I don't know why you are-- I assume it's necessary, and that you have a solid reason for doing it. Are you supplementing with hopes of weaning from supplements, or is this a permanent situation? I don't know what pace feeding means, so I'm sorry if what I'm saying is not appropriate in the light of that.
I also know that nipple confusion and preference can develop at any age, and that if babies are latching poorly, or supply is an issue, bottles can exacerbate the problem. I would really seriously consider discontinuing bottles, if you can, until you feel like nursing is going very well. If you must give one, give it in the early evenings, not at night-- the middle of the night is the peak time for milk productions. Not nursing at that time is a bad idea for your milk supply, IMO.
Both of my twins were, at one time or other, supplemented with an SNS. I haven't used a Lact-aid, but I assume it's similar-- a tube running alongside your own nipple, right? Can the flow be shut off and then turned on again? Have you tried turning it off during your active letdown, and then turning it on again once the flow from your own breasts slows? I used to do that-- baby's time at the breast increases, leading to longer feedings. But the added stimulation is good for your supply.
Switch nursing can help, too-- when baby pops off, switch sides. This might mean you're using both sides multiple times during a feeding-- this is very good for supply, since often once baby switches sides, letdown will happen again. Each subsequent letdown is fattier and creamier than the one that came before. And the more letdowns baby is stimulating, the more your breasts will get the message to make more.