Happy to answer. I don't need to "look at" why I work 19hrs a week. There's a very specific reason. As I was explaining to OP.
In order to still qualify for CA, I need to earn under £602 pcm. However if you have childcare costs you can deduct the whole amount of this from your earned income, limited up to 50% of your total monthly earnings value. Therefore, working 19hrs I take home £1029, but I'm only assessed as earning just under £600 when you deduct my childcare costs from that £1029. 20hrs would tip me over onto something like £608. That additional hour and £6 on my payslip would lose me the full amount of £332 carers a month. So 19hrs is my maximum. To work full time, 37.5hrs per week, I would take home something like £1771. No carers. To work 19hrs a week, I take home £1361. So, I'd be working double the hours, an extra 80hrs per month, for £410. My first 19hrs earn me £1361. The next 18.5 would earn me £410. It's not worth me doing that, I save it in (double) nursery fees by being at home those days.
There's an "optimal point" for everyone. And yes, I'd still earn more being full time. But only marginally more than doing 50% of those hours. And as OP says, it's quite tiring having disabled DC, so you weigh up if the small value add on is worth the disproportionately large increase in hours. I get £1029 in my bank plus CA. There are other deductions you can make from CA, I can't recall these of the top of my head, but Google will tell you.
As OP doesn't have nursery costs (I believe she said she just had funded hours so will just call it zero costs) she needs to stay under the £602 pcm to keep her CA. There's also an amount she can earn before any deductions to her UC are made, I can't remember it off the top of my head but it's around £379 as she receives rent assistance via UC. They don't deduct £ for £ it's a %age. It's more if you don't have rent assistance. So there's a number of hours/point up to which it makes absolute sense to work, and point after where it doesn't so much. OP is picking scenarios where it doesn't make sense as her examples of not being worth her working.