Unity is a really good alternative. Flash kinda went sideways and started really trying to lean into the Java crowd with flex. Unity is more like what would have happened if Flash heavily leaned into what it did well.
Unity is a bad alternative because it requires programming. It's also much more complex since it's a game engine, not a scriptable vector animation thing.
With Flash, you could just follow some tutorials and make something nice in no time, fully understanding what you're doing.
Flash didn't "lean into" Flex. Flex was an Eclipse-based IDE and a set of libraries that some people used for some projects. It was specifically aimed at building apps. There were layouts, controls and all that. There was a list view with cell reuse, in 2008! It was much more like Android layout system then what the web had and still has. Flex was a tool that's very good for some jobs and completely unfit for others — like any other tool, really.
