This is sort of backwards: "I'm a sole practitioner looking to improve my security posture by moving data files containing sensitive client data off my local machines. I currently have a standalone installation of ProSeries on Windows 11."

Nothing is more secure than something that is on your local machine, as long as that machine is under your control with security measures. Once you put something out to the Cloud/Web, now it is available simply because it is no longer local in that one place you control.

The issues for not using a relational database from Cloud storage is integrity such as record locking, or...you have the file open remotely, but you are trying to use it with local program(s) to make update(s) and any interruption or concurrency (multiple users) can create "conflicted copies" and corrupt that database (in this case, a client file). You can use it for storage, such as backup files maintenance. Example: I am on an errand while my office burns, but my working files are all in DropBox, so I can recover from that disaster.

You didn't specifically explain which part of your current system you feel is the weak point for security. Putting the working files on the web can create exposure to fraud or the opportunity for unauthorized access. It's typically more affordable to improve site security than to go out to third-party providers.