Hi, parent of 4 here.
Yep, the change from 1->2 is as hard as from 0->1. After 2+ it's log2n, not even linearly harder. The chores & experience you already have, you don't have less time ( because you don't have any anyway) and you just ruthlessly priorize between tasks. With two child you need to have a way to create free time to yourself. It's doable but it requires conscious effort, it's not the default mode anymore.
They will play with each other a lot and that gives free time back. With one child you are the only one she can count on in the early years. Sibling in my view are essential to build emotional intelligence, the skill to be able to lose and try again, cooperate and fight, and able to find a compromise.
Around 5 or 6year+ they start to actually help.
After the first child I missed my childless life. After the 4th I really don't, it really helped me grow many ways I did not thought was possible.