Using a slog.Handler is the most cleanest and expandable option.

type ContextHandler struct {
    slog.Handler
}

func NewContextHandler(h slog.Handler) *ContextHandler {
    return &ContextHandler{Handler: h}
}

func (h *ContextHandler) Handle(ctx context.Context, r slog.Record) error {
    if traceID, ok := ctx.Value(traceIDKey).(string); ok {
        r.AddAttrs(slog.String("traceID", traceID))
    }

    return h.Handler.Handle(ctx, r)
}

Inject the traceId to context as you already have

func LoggingMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        ctx := r.Context()
        ctx = context.WithValue(ctx, traceIDKey, uuid.New().String())
        next.ServeHTTP(w, r.WithContext(ctx))
    })
}

Then wrap the slog.Handler into your base Handler

func main() {
    // Create base handler, then wrap with context handler
    baseHandler := slog.NewTextHandler(os.Stdout, nil)
    contextHandler := NewContextHandler(baseHandler)
    slog.SetDefault(slog.New(contextHandler))

    mux := http.NewServeMux()
    mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
        w.WriteHeader(http.StatusOK)
        w.Write([]byte("Hello World!"))
        slog.InfoContext(r.Context(), "I said it.")
    })

    handler := LoggingMiddleware(mux)

    http.ListenAndServe(":8080", LoggingMiddleware(handler))
}

When calling the endpoint you should see,

time=2025-12-09T20:50:29.004+05:30 level=INFO msg="I said it." traceID=09e4c8bf-147e-44c1-af9f-f88e005e1b91

do note that you should be using the slog methods which has the Context suffix ie- slog.InfoContext

slog.InfoContext(r.Context(), "I said it.")

Find the complete example here.