Hi Feline83,
Thanks for your post. It sounds as though the Message app goes blank when you use dictation and when you go back to see all your conversations. We'll do our best to help out with this!
If this only happens in the Messages app, this could be related to restoring content after you set up the new iPhone. We'd suggest to connect your iPhone to a good known network: Connect to Wi-Fi on your iPhone, iPad, or iPod touch Then make sure it's connected to power. If it's restoring content from Messages in the Cloud, then that should help to resolve it after about 24 hours.
Should it persist, then we'd recommend to Contact Apple Support
Wishing you the best, cheers!