Is there a way to schedule a text message to send later on iPhone?

The built-in route is the Shortcuts app. Open Shortcuts > Automation > create a Personal Automation > Time of Day > add action Send Message, type your text and pick the recipient, and crucially turn OFF 'Ask Before Running'. It then sends automatically at that time. Caveat: it fires most reliably iMessage to iMessage, and some iOS versions still prompt a tap.

Shortcuts is the free native way and it works, just test it once, the 'Ask Before Running' toggle is what trips people up. Leave it on and it nags instead of sending.

If it is a reminder to yourself, honestly a Reminders alert at that time is simpler than automating an actual send.