Best way to backup photos

It will depend on whether you are using iCloud Photos with “optimize storage” enabled or not.

• if you are not syncing the Photos Library with iCloud Photos and Optimize mac Storage, you can simply include the Photos Library into your regular Time Machine backups and make occasionally a current copy of the complete library on an external volume.
• If “optimize storage is enabled, the copy of the library will not include all original photos. You need to export the originals to an external volume to save the high resolution versions.

Easiest is to set up time machine backing up to a dedicated external drive. Automatic and you can recover from any date going back in time (only limited by the capacity of your backup drive)

Or - you could do what I do:

I am very very rigorous about backups. Both libraries are backed up to time machine. In addition both libraries are regularly copied to a second external drive. Finally I periodically export all original images (so that I have the actual image files separately from the photos library) and copy those to a network drive backup. I have a copy of these stored away from home also (mitigate risk of fire/theft).

🙃

EDIT : PS - as leonie said - optimise storage requires additional precautions.