Hi, I expressed when I couldn't breastfeed for similar reasons, and also found it difficult to produce even half the amount needed, even with domperidone and fennel tea. Probably managed about 1/4 to 1/3, often less. The benefits of breastfeeding are mainly the immunities - ie your baby's health right now - and the possible longer term effects we hear about like IQ and obesity. Diluted, immunities are still immunities. If there's something about the long fat chains that affects the rate of brain growth in a way that fish oil doesn't, then again, I would guess that getting them at all, even in addition to something else, is going to help - like taking a supplement helps even with a poor diet. Which leaves increased risk of obesity - could that be caused by the fats in formula (eg, it is made with skimmed milk, which has been shown to have different effects from full fat milk, can't remember why) or the fats in breastmilk? The truth is, as far as we know at this point, genetics still has by far the biggest part to play in obesity, and breastfeeding is one of many elements shown to have an additional effect.
I know how tiring expressing is when you have a young baby to take care of. If you're managing okay, it will probably make you feel better emotionally to have given some breast milk, that's the truth, and as long as you're doing it, you're keeping the option open of getting back to breastfeeding, keeping your milk going, and you and your little baby may get it together with some help. But it is hard, and your baby has had six weeks of breastmilk, which is not nothing. My GUESS is that breastmilk's benefits can't be cancelled out, but I hope you work out what's best for you as well.