Something to keep in mind... do you know the saying "when your kid is giving you a hard time they're really Having a hard time?" If your child seems angry much of the time, then she's not going to be able to see beyond her own needs very well - that means she Can't be friendly and accommodating very easily. She's little. The world is big and overwhelming, and she's trying to deal with the few skills she has. And right now courtesy is just a song and dance to make other people happy - not something she can access when she's not in a good place, herself. So a big part of making it possible for her to be her best self is going to involve helping her get to that good place.
It took me a long time to realize that a big part of unschooling was actually making kids' lives easier. It seems so counter-intuitive - mainstream culture tells us over and over, that "kids have to learn" and that most of that learning involves setting them up to be challenged. But that's not how learning works! People learn better when they're the ones choosing their own challenges - and kids certainly will do that. But just like anyone else, when they're not the ones picking the challenges they're up against, they tend to hunker down and get grumpy
So it can actually be really helpful to look at what's setting your kid up to be angry so much of the time and see what you can do to make her life a little sweeter and easier. For a 5yo, there may be a whooooole lot of things that seem really trivial from an adult perspective - like not wanting to say hi to someone - that are huge sources of frustration from their point of view. So a big part of helping your kid involves looking for their point of view.
One particular aspect of making life easier for kids that parents overlook is flat out doing things for them. Again, we've gotten all these teachy messages from the world around us saying "oh, they have to do things for themselves so they'll learn". But, once again, that's not how learning works. That's something I saw over and over with my first, when we were still fumbling our way toward unschooling. When I put him in situations where he was expected to do things he didn't want to do, he didn't actually learn anything beneficial in the process. Instead, over time, he learned he wasn't a nice person But when I started stepping in and doing things for him when he wanted - even things I knew he could do - he was happier, and he learned more from the experience. His social skills were better when I said please and thank you for him then when I tried to coax him to do it for himself. So don't hesitate to help your child directly like that if it will make life easier for her - she'll learn better things from your assistance than from digging in her heels, not wanting to do whatever it is.
All that being said, I don't want to give you a false impression that making your child's life easier will turn her into something she's not. She may never be an agreeable little babydoll - she may be someone who's going to disagree with you and challenge your assumptions at every turn. And part of living happily with a strong personality is to get comfortable with disagreement and having your assumptions challenged. The great thing about strong minded kids is that they can take you places you never thought you'd go! The down side is that it's not the smoothest ride in the world