ogra@anubis:~$ snap info --verbose remmina|grep confinement
confinement: strict
ogra@anubis:~$
Not sure what makes you think Remmina uses classic confinement but none of the remote viewer/manager snaps do use classic today …
Please also note that it is a hard requirement for getting classic confinement granted at all that you pick a matching category from the supported categories list on:
some made up category like “ssh-client + remote-access-client” is not acceptable …
To my knowledge there is no category that would match your type of application though …