Photos folder on external hard drive incessant disk usage for 5 years
You’ve moved to a new Mac and a clean, maybe even a newer OS with newer versions of the software (iPhoto vs Photos).  There’s a lot that macOS is going to do with all those files in their new home on your external drive indexing, analyzing photos, facial recognition, etc.
These things are going to happen so you’re going to have disk activity.  Every time you kill these processes because you believe there’s an issue only delays and prolongs the process that needs to happen.  You have, as you say 500GB of files most likely with a ton of meta data attached - it’s going to take time to process them.
To draw an analogy, this is like seeing your car rev its engine a few RPM’s higher on start up than normal and rushing to the mechanic because you believe it’s a problem.  It’s not.  It’s behaving normally for the conditions which is probably much colder that day than normal and requiring more revs of the engine to warm up; sorta like having 500GB of files in a new location not normal and having to spin the drives and process them. Once everything normalizes, your car will rev down and so will your drives.