The problem that I am having is the DW doesn't understand the mental load thing at all and every time I try to describe it she seems to rationalise it by saying that she is too stressed/ busy, I should just tell her what needs doing, or the classic "I am not a practical person like you are"
I think the way to deal with this is that you have to accept that your DW doesn’t have to “understand” the mental load, but she does urgently need to accept that it is real and oppressive to you, and that “rationalising” it is extremely unhelpful and dismissive.
Competitive tiredness or busyness or stress is both toxic and ludicrous. Yes, everyone feels busy and stressed in different ways. Yes, this is worth acknowledging. What’s toxic is when it’s My Stuff/Stress > Your Stuff/Stress.
She does not get a free pass on ‘not being practical’ - this is a skill she can learn or improve on.
And if a sticking point is that she earns more working full time therefore is entitled to be more stressed/do less domestically/not learn or improve then you need to go back to working more and equal the financial input. Take away that argument. Improve your own position.
(I realise this doesn’t address emotions about your DC and working FT/PT but you should try to be a bit black and white in your thinking at least conceptually in order to figure out the actual issues.)