best way to back up photos

You can use iCloud Photos if you have enough space in there for it; you can create a library file on an external drive and manage it with the Photos app; you can have the library on the Mac and backup with Time Machine, also to an external drive, of course; or you can download all the pictures and place them in an external drive and manage them with folders, file names and metadata.

It's up to you to decide which one is best in your case.

My choice is to use Photos app and I pay for a bigger iCloud storage in order to store about 70GB of photos and videos.

Try creating a new library with Photos app.

Close the app and then hold Option and click to open it again. You'll be prompted with a few options, this is for you to create a new library or change existing ones.

Now you can create a new one and set the usb drive to be hit with new file.

Allow Photos to download everything there and you'll have that new library.

It might take a long time as well.

Later, to guarantee backups, you'll have to swap libraries using the "open holding Option" procedure.

Also check Download originals to this Mac in Photos Preferences > iCloud tab.

The iCloud Library is not a back up. It would not take much to erase it by trying to sync with a new empty library as part of disaster recovery.

Most Simple Back Up:

Drag the Photos Library from your Pictures Folder to another Disk. This will make a copy on that disk. It needs to be an SSD or hard disk. A flash drive is not reliable enough. It should be formatted Mac OS Extended (Journaled) or apfs.

Slightly more complex: 

Use an app that will do incremental back ups. This is a very good way to work. The first time you run the back up the app will make a complete copy of the Library. Thereafter it will update the back up with the changes you have made. That makes subsequent back ups much faster. Many of these apps also have scheduling capabilities: So set it up and it will do the back up automatically.

Example of such apps: Chronosync, SuperDuper, Carbon Copy Cloner - but there are many others. Search on MacUpdate or the App Store

Lastly, as a failsafe, uploading your photos to sharing services like Flickr or similar offer not only a last resort option but also a way to make your images available to any computer you sit down at. You can set the privacy settings on these services now to protect your data too.

Sure!

Also take a look to Yer_Man post, it might bring you some more options, since I only talked about you managing all that by yourself.