So one thing about the term “back-end” that is misleading, is that it actually really means “everything that can’t run in the browser”.
So this doesn’t mean you can’t build a GUI to show off what your learning, rather it just wont use HTML/CSS/JS in a browser. You could totally sidestep that entire environment and use something like Tkinter in Python to build a GUI. You wont be able to leverage web technologies, but depending on what your doing, that might not matter.
A back-end developer could easily setup their own website using some simple CMS system like Wix, to talk about themselves and show off their stuff. Or just don’t have a portfolio and use github as that interface. Visually you wont have much to show off, and anything you could show off, would need to be distributed thru other means.
I usually give the advice of starting “backwards”. So look at the requirements for jobs in your area for “back-end developers” or more broadly “Python Developer” (so you don’t even have to deal with the web), and see what they are asking for. What they are asking for is essentially what the “end goal” you should be going after.
So if they are asking for stuff like Django, then its still web related, or it could be data analysis, DevOps, or something else.
I’d also consider using Python as a jumping off point, and look into other languages for back-end. (Basically every language is a possibility) This also includes learning about database’s, as web stack or not, you probably will be interacting with some data from a database at some level.
If the front-end is the wild west with HTML/CSS/JS, the back-end is the rest of the world. So there is a lot to learn, lots of possibilities, lots of flexibility, and lots to choose from.
As long as you know your stuff, and know your core fundamentals for programming, and have built complex enough projects, you can break into the back-end realm. Just keep in mind what you build needs to stand out. Without any visuals to hook user’s in, your left with your background, experience, and project functionality.
Good luck, keep learning, keep building!