Adding a second iMac to my iCloud account while still being able to access all the data from the previous deceased user
I have my late father's 2021 24-inch M1 iMac which I would like to use as a second desktop with my 2020 27-inch iMac. I would like to use Universal Control between the two screens and need to change the iCloud login to my own on my father's iMac however I need to ensure I keep all his documents and spreadsheets accessible. They are mostly backed up to his iCloud but he also has a few saved in OneDrive. How can I best back all these up while keeping them accessible even though I will be logged into my own iCloud account. Thank you so much for any suggestions.
iMac 27″, macOS 15.0