I want to store some simple settings to the user’s iCloud storage, so that these settings are available on all devices. I’m talking about a few (4-5) key/value texts; not files. Is this possible?
Not currently in Xojo. Until they take the time to fix <https://xojo.com/issue/41579> anything which uses iCloud, notifications, home kit, etc is impossible in Xojo.
Ranked 41st 
Just moved it up to 30th as I need APN.
Well this is a nasty little bug! I wasn’t aware of it until now but it helps explains some issues I was having. Xojo Inc, please look at this one.