So you’re never fully committing? I understand keeping a buffer when you’re on uneven surface or if you’re riding pretty fast, but you’re telling it yourself, you’re not doing it correctly on purpose. Being able to do small circles and figure 8 backwards smoothly without leaning into it will be very difficult.
I mean you do you and keep safe! But once you actually understand riding backwards, you’ll find that’s it’s the same as going forward, if you’re falling, all you have to do is step down. The more you do it, the more you know the limits of how much you can lean back and be safe. That being said you can ease into fully committing to leaning back. You could always wear a helmet if that helps your confidence.
Good luck with your skill acquisition
EDIT: When falling backwards in general, most of us have the reflex of reaching with our hands backwards, that’s how you break your wrist or arms! I think this is more likely than hitting your head. Best is to squat down and fall on your butt and then roll on you back or sideways. Knowing how to fall is another skill!
EDIT 2: obviously a different story if you’re talking about doing backwards riding on rough terrain