I planned an only child, then had another after a big age gap. I think it's generally a lot harder having more than one child.
There are certain specific things that get easier, though--you overthink things less and your parental "investment" is spread over two children so you tend to hyper focus less. You simply have to be more practical and that narrows down choices which can be a bit of a relief. Like, extracurriculars for my eldest: she was banging on about wanting to learn the harp a while back. If she were an only child, I'd probably feel torn between "letting her down" and feeling guilty about it, versus actually trying to facilitate it at vast expense. As it is, I'm like "No, sorry. There is a baby and I can't be dragging you off on the subway to attend harp lessons several miles away every week." Decision made.
Once your children are adults, I think it's probably easier to have more than one, especially as you grow older. An only child perhaps feels more pressure to achieve what their parents hope for (or what they think their parents hope for) like getting the "ideal" job, producing grandchildren and so on. And of course, the burden of dealing with elderly parents is spread over more people so there is less guilt about that.
When they are younger though, overall it is harder to have more than one.
At the macro level, though, the move towards smaller families will probably impact parenting in some negative ways.