If I just get up and get myself ready tomorrow when DW asks if I have got DS's stuff ready, he's had breakfast etc. I say "no, you'll have to do that today" she will feel dropped in it. She isn't wrong either, she is used to me doing all of those things so I will be dropping her in it.
Rather than drop her in it, why not say the night before - taking DS to school means you will be the one getting him dressed, packing his bag and lunch etc.
This kind of illustrates the issue - when one person normally has oversight/control of a task, and then they pass over one little piece of it to someone else as a one-off, they still retain responsibility. The other person is simply 'helping out'.
I don't think it works well to try to split tasks 50/50 - so you both have responsibility for taking DS to preschool for example.
Instead, split the tasks - so you take total responsibility for school runs and packed lunches. You do it every day, you know where everything is etc.
But then give your DW a different task, for example - clothing. She takes responsibility for laundry, putting clothes away, changing DS's clothes seasonally, buying new when needed, getting his feet measured.
If you are the main contact for school/pre-school and keep track of teacher training days, parent's evenings and dressing up requests - great.
Give your DW total responsibility for doctor, dentist and haircut appointments in return.