Adopt a 3+ year old

I would go with adoption. Giving birth to another one at your age will significantly damage your health. And you will not have too much energy to suffer the first two years.
Adopt a 4 year old. He or she could play with your current child and you also passed the baby age. At 3-4 year old they are potty trained as well. You can send the kid to childcare so not much of effort. Of course, you need to be financially stable.

Two of my friends adopted one kid and give birth to one. They both said that they feel the same to both kids. Nothing is different for the adopted one.