Sonoma "locked" files and folders/permissions

Getting sick of this! My husband's less than six month old Mini M2 Pro, running Sonoma (most up to date) is HORRID about locked files/folders and permissions. He is the Administrator, and has full permission for everything.

His previous computer was blown by a power surge, and the repair place was able to recover the files from Time Machine/Capsule. They gave us said files on a flash drive. He downloaded the files, but found, upon trying to open them, that he didn't have permission. And, we were not able to change permissions. Files/folders that he could open, he was not allowed to make changes and save them because he didn't have permission. Yet, when looking at Get Info, he DID have permission. Unlocking the files/folders and changing permissions for Everyone (allowing all) didn't help, either.

The solution, for the meantime (he's trying to do taxes!) WAS to open a file, then save it with a slightly different name, onto the desktop. That, however, does not always work. Sometimes THOSE "new" files lock themselves "automatically". WHY? Again, trying to make changes and saving doesn't work, so once again he has to save the file with a slightly different name, into a different place.

From what I've read on the Internet, this seems to be some issue with Sonoma, and has been for some time, yet I haven't found any solution that works (nor have others from what I've read). He is working with Excel for Mac, but it doesn't seem to make a difference what the software is - I've read that this happens with Numbers, Pages, etc.

So, Apple, in your infinite wisdom, WHAT is the solution???