Ok, so regardless of diagnoses or not, you need strategies.
‘Mummy … mummy … you know mummy … mummy, you know, you know Mars is, mummy, it’s, Mars is, mummy, it’s not hard, it is not hard mummy …’
"Sam, take a deep breath, I'm listening. What do you want to say about Mars?" When he's finished "Oh, so Mars is <insert key phrase from his rambling>? That is interesting. Ok, let's read the next bit."
Sometimes they just need time and help to organise their thoughts.
The hitting:
The Red Beast https://amzn.eu/d/07MSJ5Pt is a really good story book. It talks about anger being like a red beast. As he 'wakes up' (gets angry), his eyes get smaller, so he can't see clearly. His ears get smaller, so he can't hear, and his mouth gets bigger, so he shouts, etc.
Does your DS seem worse when he's hungry or thirsty? DD3 used to get 'hangry' and when she was young we made an agreement when she was calm that if I said 'eat' or 'drink' she had to eat or drink whatever was put in front of her. I used to give her some orange juice and a digestive biscuit. I'd literally say 'Eat' and offer the biscuit to her lips, then 'Drink'. Within about 10 minutes she was calm. She just couldn't regulate her emotions.
Messy eating - try to pin down why. Is he distracted by his environment so not paying attention? Is he over excited so being impulsive/silly? Is he not aware of how hard he's holding something so it crumbles and crushes? Does he struggle to sit still so he's knocking food/drink over? Does he struggle with his motor skills? Is he clumsy so he's dropping it? Is he not very good at knowing where his mouth is, so he's missing it and getting it on his face? Does it feel nice to have sauce/chocolate around his mouth?
In terms of your relationship - go easy on yourself. It's intense and annoying. But also go easy on him. He's probably not just irritating you for the sake of it. Something is causing this, even if it is just immaturity.