Re: Migration from Dropbox to iCloud or another service, as attorneys I assume both @katiefloyd and @MacSparky have an Office 365 subscription in order to use the MS Office suite of applications, which are standard in the legal and business communities. I have one, and I get 1 TB of OneDrive space as part of the plan. Why not use that for file storage and sharing? You’re paying for it already.
In my case, I had a Dropbox account as well as my OneDrive and iCloud accounts, but abandoned the Dropbox account in order to simplify things. I simply didn’t need 3 cloud services, so I migrated my personal files to iCloud. Now I keep my personal stuff, photos, music, etc. in iCloud, and business, work, and client-related items in OneDrive.
OneDrive is very good, and I suggest you try it if you have not already. It has an iOS app and a Mac app that allows me to sync with all my devices (iPad, iPhone, iMac, and PC at my office), and allows me to set up a shared folder with clients. The iOS app integrates with the Files app, and the Mac app integrates with the Finder. It’s been very reliable on all platforms and is regularly updated with new features and performance upgrades.