Hey allykay18,
Thank you for answering those questions. From what you're describing it sounds like your photos haven't finished downloading to your iPhone. When you disable Optimize iPhone Storage the photos are required to download to your device via WiFi in the original quality. This can be a lengthy process depending on internet speed and the amount of photos that you have.
In having and iCloud back up from your iPhone 6, I'd recommend performing an erase all content and settings on your iPhone SE 2nd Generation. After the iPhone SE 2nd Generation has been reset you will want to transfer the information back onto the phone using your iCloud back up from your iPhone 6.
Here are a couple resources that will help:
Erase all content and settings from iPhone
Transfer data from your previous iOS device to your new iPhone, iPad, or iPod touch
After you have successfully gotten the information back onto your iPhone SE 2nd Generation you will want to select Download and Keep Originals when disabling Optimize iPhone Storage.
Here is a great resource that will walk you through that process:
Get help with iCloud Photos
Specifically, this part:
"If you have Optimize Storage turned on, you might not have all of your original photos and videos on your device. To download the original photos and videos on your iOS device, go to Settings > [your name] > iCloud > Photos and select Download and Keep Originals. On your Mac, open Photos, choose Photos > Preferences, then select Download Originals to this Mac. Then you can turn off iCloud Photos. You can also select the photos and videos that you want to download from iCloud.com."
Take care