lovinit, the short answer to your question 'Would it make sense for me to use a formula feed at 5-6pm and then express for the 10pm feed once milk supply has built up ? ' is no
In fact, not only does it make no sense, you are reducing the amount of milk available to your baby.
I have explained a few times (more than a few!) how this works - the longer the gap between feeds, the less milk you make.
This is how weaning works - what are women told to do, if they want to switch from breast to bottle? To replace a breastfeed with a bottle, and then after a few days to replace another bf with a bottle, and so on.
At first, a dropped feed (in your case, the 5-6pm one) will lead to a fuller feeling at 10 pm. This is all it is - a feeling. There may be a bit more milk there, but not a huge amount. After a few days, this 'bit more milk' will cease. Having a 'bit more milk' does not mean your baby will take any more. Why would she? She will take what she needs.
It makes even less sense to express at 10 pm. You are then risking less effective stim. of the breasts. The baby is better at this than the pump.
The time your baby is on the breast is normal.
lovinit, you don't have a problem with supply (though you will if you start dropping feeds). You have a problem with confidence in your milk supply.
If you need more explanation of how frequent feeding leads to more milk, and how lessening the frequency leads to less, then look back at the other threads. I don't know how I can convince you any more!If your confidence is not there, you ain't gonna believe it!