this is best if you turn it into your cold storage otherwise just encrypting the USB is not really giving you much of a security in my opinion. and to do that you need to disconnect your network inside the installed OS so that it can never connect to the internet.
you might also want to disable some extra stuff including guest login,...
as for the choice of OS i personally dislike anything that is not popular (i haven't heard of) and since i am barely familiar with Linux i prefer sticking to the popular ones because it is always easier to fix a problem with them since they also have the biggest communities.
in this case an alternative to AntiX could be different flavors of Ubuntu. for example the most similar one would be XUbuntu which is using the same DE called Xfce so it is similarly light and can be used on older computers while it has all the features that come with Ubuntu.
encrypted USB is physically more secure and yes of course if use it for cold storage you should disconnect from network.you might also want to disable some extra stuff including guest login,...
as for the choice of OS i personally dislike anything that is not popular (i haven't heard of) and since i am barely familiar with Linux i prefer sticking to the popular ones because it is always easier to fix a problem with them since they also have the biggest communities.
in this case an alternative to AntiX could be different flavors of Ubuntu. for example the most similar one would be XUbuntu which is using the same DE called Xfce so it is similarly light and can be used on older computers while it has all the features that come with Ubuntu.
AntiX full disk encryption is so easy for beginners, others need some commands and partition configuration.