I could move my .photoslibrary file to an external hard drive and choose the "Download Originals to Mac" option to insure that all photos are stored locally. However, this would defeat the ability to pick up my laptop and use my photos on the road.
My best option to date is to export the photos by year. I create Smart Albums, one for each year.
Every January, I then export that smart album to my external hard drive. I also keep a tally of how many photos are in each year's backup album. This helps me keep track if I add or delete old photos. The system is not automatic, and I wish it were, but at least I have some confidence that if my photos library crashed either from an Apple error or my own stupidity, I can re-create it later.
The Export function gives me the option to export originals, or my modified versions, and for many of those old years, there's no difference, but it is an issue that I haven't resolved. Starting an Export will force the Photos App to load any of the photos that were staged in the cloud, so there is a wait time involved with that, but it does the job. Again, I wish there were an automatic backup function that would monitor any changes and allow incremental updates.