Best way to handle embedded sub expressions / language

[vikigenius]

I am struggling with why Rowan does not allow payloads in the SyntaxKind

Consider this language Yuck, it's based on lisp, but it has an embedded sub language https://elkowar.github.io/eww/expression_language.html
You can only put this within Curly Braces something like: (box {button_active ? "disable": "enable"})

if payloads were allowed you could easily do something like

enum TokenKind {
    // snip
    SimpleExpr(Vec<SimpleExprTokenKind>)
}

But since Rowan can't allow that we just have

enum TokenKind {
    // snip
    SimpleExpr
}

You would have a separate stage to lex and parse the embedded SimpleExpr, but what if there is a missing closing brace? How would I handle lexer errors in the sub language?

I would appreciate any suggestions/ideas on how to best handle this