When you say you want to give more to Charli are you talking in an emotional sense or material? Or both? If you're concerned that you can't love a second child as much or a second child will reduce the love you have for Charli I don't think that you need to be concerned - love isn't like a cake that you have to divide. When a new baby arrives you get a whole new cake.
From a material POV it might be true that you will be able to buy less for each child but they will be able to share many of their possesions so maybe it needn't be a concern.
I see my 2 playing together and I think that having a sibling is a fantastic gift even though they sometimes squabble. Yasin hasn't missed out on anything because he has a little brother, on the contrary his life has been enriched.

I don't think that children are disadvantaged socially by being only children, many parents of only children offer them a rich, socially fulfilled life.