I don't quite understand why you're being asked to top up. If baby is finishing feeding on their own terms, then why would they want a bottle afterwards?
If you are keen to express to make the bottles, I would wait 30-45 minutes after feeding (perhaps while baby is sleeping) to express. Because we just don't have enough to do each day already. But for the first day or two that might exacerbate the problem at the next feeding time (ie. insufficient milk) until your supply catches up. (NB. I expressed full time for 12 months, mainly while DS was asleep or on the floor playing.)
Was the baby big at birth? Are you/DH small? Is there any possibility they are just moving down to a natural, comfortable size?
As long as the wet nappies continue and you've ruled out reflux etc, I think continue doing what you're doing. Maybe instead of offering a bottle put baby on the breast more often to see if they'll feed more.
DS was heading towards being underweight, and also dropped from 50th to 10th centile, but he had very bad reflux (fortunately he was a 'happy chucker', so we didn't have the screaming and pain associated) and so we knew the reason he wasn't gaining enough weight was because he wasn't keeping any food down.
I don't want to frighten you, and it is unlikely, but it is possible that your milk is not 'good enough' and you may need to move to formula. I'm not a formula advocate by any stretch, and would only do this as a last resort of a GP or paed determined that baby really, really needed it.
Best of luck with it all. It's hard at times, but I bet in 12 months you'll be lugging around a heavy beast of a baby wondering what all the fuss was about.