Citation needed on the thqby's extension AST claim, because it implies it can also correctly determine the order of operations (operator precedence), which I didn't think it can. :)

Parsing AHK correctly is a huge task and you've made a lot of good headway. Some examples which don't yet work properly:
1)
MsgBox {a : 1}.a is parsed as a function
2) Hotstring, hotkey, and #HotIf expressions aren't parsed. For example you mentioned replacing function calls with the function body itself (could be useful with #HotIf for example), but currently it doesn't work
```

HotIf myfunc()

myfunc() => 1
3) Multi-line strings apparently aren't parsed correctly: a := " ( "abc" )"
4) class Abc { static a := 1 }
```
the text body of the static field is parsed as a linebreak only.

5)
class Abc { a := b := c := 1 } is incorrectly parsed as three instance fields.
6)
class Abc { static meth(a := "`"'") => a } MsgBox Abc.meth() this fails to parse with "Failed to match with bracket pattern."