Did not intend to turn on iCloud Photos! Want to undo!

In my 10 years of being an iPhone user, I have never used iCloud Photos..preferring instead to keep all my photos on my device and download to my computer periodically. This system has worked for me.

I have just upgraded from my 5-year old iPhone 11 to a new iPhone 15, iOS 18.0.1.

I successfully restored my iPhone 11 backup onto my new phone, and in the process (perhaps it's the default setting?) iCloud photos was turned on and my phone has synced to the cloud.

It's my understanding that the original, high-res versions of my photos now reside on iCloud, and "space saving" versions are what I see on my phone.

Ultimately, I want to delete everything from iCloud Photos and stick with my tried-and-true system of keeping my photos on my phone only. I'm not confident on how to do this without losing the high-res versions or, worst case, complete files altogether. I'm not brave enough to DO anything without wise counsel, but this is what I THINK:

Under Settings\iCloud\iCloud Photos, currently "Optimize iPhone Storage" is selected. Should I change this to "Download and Keep Originals" and then once that process is complete, UNsync the phone and then delete the files from iCloud?

I am bleary-eyed from attempting to research the best solution. Help, Apple Community!

Many thanks in advance.