I am using island on my (non-Planet) tablet and it works as expected, no major problems. On my Cosmo I do not need the sandboxing, but just out of curiosity, I tried both shelter and island and while they work, reclaiming resources (i.e. killing sandboxed apps) kept killing my mainland termux session (no matter what I did) so I uninstalled them.
Another way of separating would be to create multiple android users (available on Cosmo, but probably not on 7.* Gemini), but this has two major drawbacks - switching the users takes some time (half a minute or so), and background processes of the other user(s) keep running and occupying the RAM, even if I use the other user only occasionally.