Apologies in advance if this is a slightly harsh response, but:
As a single mum with shared care of my kids, I don’t expect their dad to look after the kids at times they’re set to be with me. If I can’t care for them myself I do what most single mums have always done: call in help from friends, neighbours and family when possible, and pay for childcare when not.
You mention your ex is on a lower income than you: I have no idea but I wonder, if she’d spent the past 14 years building her dream career full-time would that still be the case? The comparisons you draw should not even be relevant to the question of how you take responsibility for your childcare.