You sound burnt out. Not surprisingly.
The way you speak and the way in which you have in some ways resigned yourself and you're not fighting them occasionally means that you're at the stage of preserving the very little energy you have left. I mean this very factually, I'm not judging you. It sounds pretty hellish.
Objectively, you need to recover and the only way I see it is you farm out one of them as much as possible to your partner, extra childcare, extracurricular activities. It's not bad parenting, it's recognising you need to recover and have better physical and mental capacity to deal with them.
Another thing is, farm yourself out. How much time to yourself do you actually get? Because it sounds like you have to make it, as a non-negotiable. You are drained and overwhelmed at the moment and you need to recharge.
As bad as this may sound, I'd also leave them to it absolutely as much as possible. If it's not unsafe for either, leave them alone. There's only so much one can take in terms of managing them.
I think part of what you're describing might be age gap, boredom (for the older one) or the fact they are competitive or have too much energy. If they're not ND, it's probably a mixture of all. And of course it's massively hard on you, this is completely understandable. I hear you, OP 💐