ZFS will use memory if it’s there but isn’t essential. It’s well documented but that doesn’t mean everyone will do your sysadmin for you. There are ways of expanding, but not to overcomplicate you should consider the only straightforward way is to make a new array on other disks and copy things over (or, for just one reason you might use this other than mdraid, replicate a snapshot to it).
I’d suggest an LTS release of ubuntu for a server, even a ‘server’ (9 month release cycle on 24.10 et al). If you were not planning to sit in front of it and use it I’d really suggest truenas as it has a UI designed for doing these things, leveraging zfs’ features while abstracting away some nuts and bolts, and not easily letting you do something terrible without it telling you. On ubuntu expect to have to use the command line at points.
Build an array to play with, on old laptop drives or something like that, and be sure you’re happy with what you understand of how it works. ‘your’ data is the only part of IT that’s not replaceable.
Here’s an overview that, i’d guess is better than anything you read yet.