iTunes requiring new phone set up, but it's not a new phone

In January I switched over from my iPhone 6 Plus to my iPhone X. To transfer all my information from my iPhone 6 Plus to my iPhone X, I did a backup of my 6 Plus onto my computer, which is what I've always done, and restored my new phone from that back up. I had all my photo's and apps, but none of my music from my iTunes library. I thought that was weird but eventually forgot about it.

Fast forward to about a month ago, and I remembered that I hadn't backed up my phone since I had restored it in January. So I bring up my iTunes and plug in my phone and the message "Welcome to Your New iPhone" pops up... except this phone is definitely not new. It gives the options to either "Set up as new iPhone" or "Restore from this backup" giving the date of when I had last backed up my 6 Plus, which is how I got all of my data from my old phone to my new phone.

I'm afraid that if I use the "Set up as new iPhone" option all my data will be deleted, and if I use the "Restore from this backup" that I'll lose all the pictures and things I've accumulated since January. So my question is, how do I fix this? Is there any way for me to connect my iPhone X to my iTunes and be able to back it up without losing any of my data?

Both iTunes and my iPhone X are up to date. iTunes version is 12.8 and my iPhone X is running on iOS 11.4.1.

This is the screen that pops up when I plug in my iPhone X to my computer for reference.