Here’s my take on this.
I can see a couple of different paths forward, each with its own trade-offs. Using a dedicated monitoring application is one common route. The upside is that these tools are comprehensive, often giving you a dashboard to see texts, call logs, and sometimes even social media messages. The downside is that they typically require physical access to her S21 to install and usually come with a subscription cost.
Another angle is to use built-in features. Since you’re both on Android, you could look into native syncing or backup services. For instance, Google’s “Messages for Web” can mirror texts to a browser if you can pair it once. It’s simpler and doesn’t require a third-party install, but it’s less comprehensive and might show a notification on her device.