Hi Swartkat
It is normal for babies to want to clusterfeed during that time. The way I tried to think about it was to that baby makes his tummy very full then, so he can get a good long stretch of sleep, and so can you (hopefully!).
I would try to avoid giving formula, it a bit of a downward spiral when you give formula, you don't breastfeed, which signals breasts to produce less milk, leading you to give more formula, and so it goes. Your babs is gaining so well, he definitely gets enough milk from you, you don't need to feed him formula.
My second LG was also a champ feeder, at 5 months she doubled her birth-weight just on breastmilk. The International Board Certified Lactation Consultant at my local Breastfeeding Clinic assured me that you very rarely (like never) get an overweight exclusively breastfed baby. EBF babies regulate their intake better. In addition, the "fat" from breastfeeding is good healthy fat, so do not be concerned about overfeeding an EBF baby.
On expressing, I found that the easiest time to express is after the first and second morning feeds. Milk supply is usually more in the mornings. If you express right after the feed, your breast will have more time to fill up for the next feed.
It does not sound like you have an issue with supply, but if you want to increase it, try eating oats. Just normal jungle oats, or even the oats bars are great as a snack for a hungry breastfeeding mom!