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That's a good idea! I'm quite sure that all data is encrypted at rest by default on all M* Mac models so all you'd need is a second system partition. Never mount them at the same time and you'll have acceptable protection on anything that doesn't have chip vulns. So M3 and up is fine right now.
Yea the only obvious hole I can think of without thinking too much about it would be something infecting the boot loader, a physical switch would prevent that, but there's probably other ways to mitigate.
Yeah. Privilege escalation could turn off secure boot - at least all I needed was admin to turn it off last time I tested that. So that risk remains, but it is pretty obvious that this is happening when you boot, because it tells you and iirc it doesn't auto boot then.
Same software surface risk you face when downloading pretty much anything that talks to the internet
If the device you would use it on has co-existing data you don't want to exposed to that surface you could boot into separate images, one for your stuff one for when it's idle... having your personal one encrypted.
That's not full isolation ofc, but about as good as you can do without a physical disk switch.