I'm just wondering whether anyone else has found that their eye colour has naturally changed over time (not as a consequence of HRT).

I started out with light blue/grey eyes but over the years they've become increasingly hazel to the point I'm no longer sure which I should describe them as XD

This is really common in my family. It's subtle for some, like how I started with deep, dark blue and these days it's more like a pale blue/grey color.

My little brother had brown eyes, and now they are blue. For a long time one of his eyes was actually both, a quarter of his iris being brown, as the rest gradually went blue.

they were a lighter brown to a deep brown (but I'd rather say that they are black lol even though they are not)

I don't know if mine have changed , they are more like a chameleon , they seem to change color depending on when and how I look in the mirror , usually they seem brown , but in closer look and with natural light , they are closer to green .

Mine shift between grey and blue...  Right now they're a bluish grey.