changing computers, want to be sure I do not lose my photos

If you have iCloud Photos turned on, use any computer internet browser and log into www.icloud.com and go to the Photos app there and verify that the number of photos on iCloud (at the bottom of the screen in the Photos app) agrees with the number of photos in the Photos app on your Mac.

You can erase your Mac without affecting iCloud by using the following procedure: What to do before you sell, give away, or trade in your Mac - Apple Support

However, personally I would also want to have a local backup to a USB drive at all times even if I wasn't moving to a new Mac.

In your old Mac if photo library and its preferences is turned as on https://support.apple.com/en-in/HT204264

https://support.apple.com/en-in/guide/photos/pht5156cc968/4.0/mac/10.14

https://support.apple.com/en-in/guide/photos/pht211de786/4.0/mac/10.14

When new Mac is set up https://support.apple.com/guide/macbook-pro-16-inch/set-up-apd831707cb3/2019/mac/10.15.1

And you use the same Apple ID and password of your old Mac in this new Mac and set up iCloud , iCloud Drive see the articles  https://support.apple.com/en-us/HT208682

And then set up iCloud Drive https://support.apple.com/en-us/HT204025

The original photos with full resolution will once again sync , some essential conditions , your iCloud storage data in previous Mac should be in designated limits it could be free 5GB or the next storage plan that you purchased and used .