At least go to "classic". I don't want to sidestep the issue with links or bindmounts...
As a user, you have a limited amount of flexibility over how you want individual snaps confined. You can install a
confinement: strict snap in devmode, for example:
$ sudo snap install --devmode <snap name>
You can also install a
confinement: devmode snap in "jail" mode (i.e. treat it as strictly confined):
$ sudo snap install --jailmode <snap name>
What you cannot do, however, is install a
confinement: classic as anything other than classic, or similarly a
confinement: <not classic> snap as classic.
This limitation isn't to be annoying, it's because
confinement: classic snaps are built entirely differently than
confinement: <not classic> snaps, and in virtually all cases running them under another model would result in a segfault or something similar. This has to do with the fact that
confinement: classic snaps do not use the
core snap as its rootfs/execution environment like the other types of confinement, so it finds things like libc in a totally different place.
In general, I don't recommend taking advantage of the ability to install snaps under different confinement models unless you're utilizing them as a developer testing out your snap. Otherwise you'll be using the snap in a way that is probably completely untested by the snap author, and may or may not work properly.