As i said, there is no way to influence classic or strict on the user side, this decision is made at package creation time, classic snaps have to be created completely different to strict snaps and undergo a completely different release process with manual reviews and deep security inspection since they are more powerful …
you can see the criteria a classic snap needs to fulfill here:
You can see there that classic confinement is also limited to a very narrow set of applications (see the list of Supported types on that page) where strict confinement is simply impossible and would completely kill the functionality of an app (i.e. IDEs that need to have access to header files in random and unpredictable file system locations).