Oh I can totally relate to this, because when I was pregnant with my second, my breasts also felt so full and uncomfortable around that stage, and I remember being quite worried too. With my first I had also exclusively pumped, so my body already "knew" what to do and started producing earlier the second time around.
From what I experienced, some gentle hand expression to relieve the pressure is something a few mums do, but I would really encourage you to check with your gynae or PD first before trying anything, just to make sure it is safe for your current pregnancy. Some doctors are cautious about nipple stimulation during pregnancy as it can sometimes trigger contractions, so it is better to get the go-ahead from your doctor before you start harvesting.
You are doing so well already thinking ahead for your baby, and I hope you get the answers and relief you need soon.