How to transfer files and photos from a deceased user's account to a new user account on a Mac mini?

Legacy Question

Dad passed 4 years ago. He was the tech savvy one and Mom is definitely not. She even relied on him for printing. For practicality's sake and to keep it simple for my grieving mother I decided to keep Dad's account as is on the Mac Mini. After all, he managed all the logins, filed all the (digital) paperwork as well as the family photos (locally not in the cloud). Since then, Mom has come along way and learned to get by, but mostly relies on her iPhone and iPad. Some tasks are just easier on big screen and on this holiday visit I realized this strategy might not work any longer.

I noticed her email hadn't been accessed on the Mac in 2 years. The connection was broken and it was a major pain to try and reconnect them because the built in password keeper software really only transfers stuff seamlessly between iPhone and iMac when you're the same user (obviously). I've decided its time to open Mom her own user account on the iMac and migrate/transfer as many of the files and photos as I possibly can to her side so we can finally put Dad digitally to rest.

I'm seeking suggestions, advice, support articles, and warnings (about pitfalls to avoid) - on how to accomplish this easily (and preferably simply). Thanks in advance to the kind souls willing to put forth suggestions- rather than telling me all the stuff I did wrong - had they been in my place. Thank you.

[Re-Titled by Moderator]

Original Title: Setting up a