If I'm honest, and this is for me personally, I don't find the mental load of having a kid too bad. Most of my mental load would be there regardless. If I look at my to-do list the kid-specific stuff is pretty minimal compared to a demanding job, retraining for a new career, and having two autoimmune diseases. My kid is healthy and doesn't require any regular medication or appointments so I think that does make a difference, but whether I had a child or not I'd still need to buy food, keep the house clean, go to work, manage my health, pay bills etc. And the few kid specific bits of mental load I have I honestly don't begrudge for the sheer joy my kid brings me.
Honestly the things I find hardest mental load wise is managing the health stuff- because I can end up having to chase between the hospital and the specialist pharmacy and the hospital no longer have any phone numbers you can contact, and I can only ring round places one day a week because I can't even look at my phone in work let alone try and get something non-work related done.
I don't sign off from work stress at 5pm on a Friday, it lingers, normally till 9am on a Monday, and then I look at my emails and the next week of stress starts. Signing off from work on time would be a dream! I don't get two weeks annual leave twice a year because a lot of my leave goes to my kid being sent home sick, or just having to take days to get things done here and there. I'll try and take a day or two for myself around my birthday and I like to have a day before Christmas where I go to town but mostly leave needs to be used for practical things. The difference between mental load and a full time job is that if you have a full time job you have both... just less time to deal with the things that result in mental load.
The other week I took two days off where my kid was in childcare because I had college work, hospital phone calls to make, and some areas of the house needed the kind of cleaning I can't do while a kid climbs my back shouting "giddy up mummy horse!". I planned out each hour of the day, alloted time to each task I needed to complete, got all my jobs done for the day with a bit of time allocated to myself as well and then picked my kid up a little early each day so we had a more relaxed bed time and OH MY GOD. It was so relaxing, just being able to go through and get things done... I did zero wanks, chazzas or pastries, just focused on getting things done. Maybe if I really want to be a martyr to mental load I need to fit more wanks, chazzas and pastries into my day.