Focus on one thing at a time and pick your battles.
Getting dressed
Lay out two outfits the night before and let him mix and match what he wants to wear. He chooses and gets dressed or you do it all for him.
getting him out the house
Sometimes you just need to bodily grab and get out the door. If you're cool and calm about it and show you're unbothered, his fits will stop having any power and he'll stop it.
getting him to eat anything sensible
Definitely not your problem to solve. Serve up food and he eats what he wants and however much he wants. If he gets a yoghurt or piece of fruit after dinner usually then just stick it on the table at the same time. No pleading with him, no trying to convince him to eat or bargaining with him. Just "there you go". If he refuses to eat it, fine. Let him go without.
I tried enticing him to go in the garden this morning but we only managed a few mins before he said he wanted to go back inside
You might need to set up actual things to do and start doing them. Set up an activity in the garden and do it together. Something like a teddy bears picnic or a game of hunt and find is more fun in the better weather. Inside the home you can set up "invitations to play". Before I go to bed at night I often set up a little scene of some sort to get my son started. When he sees it in the morning he carries on playing with it for half an hour or so. Example, box of magnetic tiles with some built as ramps with a basket of little cars. Or the sand pit set out with a basket of shells and various things to scoop and dig with.
To top it off today I broke down crying in front of him because he wouldn’t brush his teeth and was demanding his Easter chocolate
Don't worry about this affecting him. We all snap and shout or cry sometimes. They push us beyond our limits. As for demanding the Easter chocolate I would said "No we're not having any chocolate right now". If he screamed and tantrummed I would say "mummy said no" once more and then completely disengage and ignore the tantrum. Happened today with my son. Screaming for a flapjack. Two "nos" and then I ignored him entirely. He stopped eventually. His tantrums only last a few minutes because he knows they don't work. As soon as he stops be ready to be bright and breezy immediately. "You're all done being sad now. Great! Look I'm building a Duplo house"
I would leave the potty training for now and use the holidays to practice getting back in control. He's seeing your moments of upset and weakness and running rings around you. Once you're feeling more up to tackling it, return to the potty training. I also highly recommend bratbusterparenting on Instagram and YouTube.
Good luck!