Hello AwesomeIphoneF,
Thank you for posting in Apple Support Communities.
This is definitely something you will want to look into prior to getting a device with a lower amount of storage.
To start, follow the steps found here to see how much of your storage is currently being used: How to check the storage on your iPhone and iPad - Apple Support
This will tell you the amount of storage you will need on the new device. If it close to or above 128 gb, you will not be able to add everything from this phone to the new one.
Using iCloud for your photos is helpful but will not ensure you can fit the full back up on the newer device.
As you can see here, What does iCloud back up? - Apple Support , anything that syncs with iCloud will not be included in the backup for that device. This means if your photos sync with iCloud, they will not be included in the backup. Even if you can fit the backup on the new device, you will need available storage to allow access to data that syncs. For example, photos. The photos will be stored in iCloud, but you may not be able to access them on the new device.
We hope this helps.
Kindest regards.