Migrate data from old iMac to new Mac Mini and move all files (documents, photos, etc) from the old admin account to a new account

At this point I would not attempt a "do-over."

You can batch change permissions.

Do GetInfo on a folder whose contents you want to change permissions for. Then open the Sharing & Permissions panel and click the padlock icon to enable you to change permissions. Change the permissions for Everyone to "Read and Write." Then click the little "circle with three dots" icon at the bottom of the panel and select "Apply to enclosed items." You will get a warning that this cannot be undone but since you want to change the permissions on all the files & folders inside the folder in question, go ahead with the operation.

Depending on how your folders are set up in the old admin account you may find it better to log into the old admin account in order to rearrange those data folders in order to make batch changes easier to do.

With "Everyone" having Read & Write permission for each folder and file you should not have any trouble moving/copying/opening the files in any user account.

If you are not comfortable giving Read & Write permission to "Everyone," you could instead just change Admin to "Read and Write" in which case all Admin accounts would have access.