Reset my phone

If you value your data, you need to save it somewhere other than the phone before resetting or trading in the phone.

If some of your data is synchronized to iCloud (e.g., iCloud Photos), or is saved in backups, that data could survive a reset of the phone.

If you can delay returning the old phone (and getting the trade-in credit) until after you have set up the new phone, that would give you a chance to check that things made it across, and might also let you use Quick Start.

Use Quick Start to transfer data to a new iPhone or iPad - Apple Support

Then you could prepare the old phone for trade-in.

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

You will lose any information that is not stored somewhere else. Your iPhone will be wiped clean.