iPhone 18 Pro photo editor missing pen, pencil, and eraser tools

The Pencil, etc. are not a part of the Tools menu in iOS27. The tools are mainly showing the artificial intelligence tools.

You have to use the Markup extension. See: https://support.apple.com/en-us/119875 (in the paragraph "Photos"):

1. Go to Photos and select the photo you want.
2. Tap the Edit button. 
3. Tap the Markup button. Tap the Add button to add text, shapes, and more.
4. Tap the Done button, then tap Done again.

When only the one item is shown at first, the menu with Markup tools has been collapsed. Click this one item to reveal the complete list of all tools.

This seems to be a an unfortunate, unintended behavior.

It sounds to me like you are correctly choosing the Markup tool, but you're seeing this:

with only one pen visible-- is that right? That got me the first time.

You tap that and it opens the full set of tools:

Does that work for you?

By the way, turning your phone off and on doesn't really do anything at all. If you want to reboot the phone try a forced restart: 

Force restart iPhone - Apple Support

This is different from a simple Power Off. For me, the logo appears  5 or 10 seconds past the "slide to power off" message. Wait for the logo; don't do the slide thing.