Do you have any evidence supporting this?

Sorry, I missed the “first years” portion of your comment. I had originally read it as simply paying people to be stay at home parents for the whole time they have children at home.

Paid parental leave up until at least breastfeeding is done is a no brainer.

Just spitballing, here, but maybe some sort of voucher system to distribute leave among all your "alloparents?" But that's hard to systematize. Probably the simplest thing is simply to increase wages and PTO across the board, and let workers/consumers act on their own priorities.

no need for vouchers. we just need the right and the support for more people to work part time. that right for example is already law in germany. the problem is that not many people make use of it, partly because it would reduce their income but partly also because there is no broad social support for people to reduce their worktime in order to do other useful things.

actually it is possible: education. basically, the reason one might not want some people to have kids is their lack of education. provide them with better education and that "problem" goes away.