Backing up an iPhone without the use of a cloud service or a computer I will soon lead a lifestyle where I'll be required to travel a lot, and at the same time I've decided to ditch (or at least just phase out using) my computers (both Windows and Mac).

I will be using my iPhone from now on, for all things digital, so I'll move all my files from my computer to my phone.

That sounds all good in theory, but given I do not use any cloud services (nor do I plan to), how will I backup those files?

So far, I kept all my files on my computer. I'd just plug in an external HD and use either Time Machine (on Mac) or an encrypted VeraCrypt container (on Windows) to backup my files.

What can I do to backup the files on my iPhone, without a computer or a cloud service?

While, as of recently, iOS can mount an external flash drive, it makes no sense tranferring my files to an unencrypted drive just for the sake of backup. (Correct me if I'm wrong, but it can't mount an encrypted HD, does it?)

Can you think of any solutions to this problem?
iOS only supports backup to a computer running iTunes or to iCloud.

If your aim is to keep everything on servers you control yourself you might use an NextCloud installation (with disk encryption) and use the Files app (or native NextCloud apps) to manage your data. That way you at least won't use your data in case your phone gets lost or stolen.