There is no version beside that one in beta available.
Strict confinement means that the application is isolated quite well from your system. Think of it like the mode apps use on smartphone operating systems, such as Android or iOS.
Devmode usually means that the developer hasn’t adapted or configured the application to work well in this confined mode. In devmode, the application has no restrictions, which might be okay if you trust the publisher and don’t need automatic updates etc.
See the GitHub page for detailed instructions on how the author expects you to install and update the snap: https://github.com/Thermionix/fusion360
In short: sudo snap install --beta fusion360 --devmode should do the trick for installing the snap.