This is based off the expressing. I only express 1-2 mls at the moment but I am pretty sure baby is suckling more milk out than the pump. We also know I am not producing a large amount as baby lost 600grams in 4 days when I was exclusively breast feeding him. He also became dehydrated and looked very sickly.
Ok. Now we are getting somewhere.

Feeding him 8 times in 24 hours is great. Keep aiming for 8, up to 12 feeds in 24 hours, the more the better. Sounds like he is having pretty good output too.

Pumping is not a good indication of your supply. You're right - baby will be suckling more from your breast than the pump is able to extract. Also - pumping straight after a feed is not the best way to get milk out. Wait for an hour if you are able, and pump then.

The best indication of your milk supply is your baby. How many feeds in 24 - like I said, aim for 8-12. Wet and dirty nappy count - 5-6 or more in 24 hours is great. Growth - not just weight, but length and head circumference. General appearance and countenance - is baby relatively content?

It sounds like bub is ticking a few of these boxes now, so you might be able to cut back on the top ups and see how you go. The tricky thing about bottles is that baby's natural instinct is to swallow whatever goes into their mouth. Survival instinct - swallow or gag and spit. So even if he seems to be guzzling the bottle, it doesn't necessarily mean he is hungry.

How would you feel about reducing the amount of top ups?