I have never liked carrying crossbody bags either till the last couple of years (I sold off most of them years ago). I am looking forward to using it (trying not to worry about possible color transfer, I wear jeans but not dark ones or new ones

.).
Yep, the strap is non-adjustable like you said. It hangs just a little lower than my Alma BB but it is fine for my height ( 5’ 3”). I have a generic adjustable strap (for the Speedy20) that I can use with it if I want.
I can fit a mini pochette in it. Along with a ZCP and 2 key pouches and still has some room left. Not sure if a sunglass case would fit on top since the shape is like a trapezoid (tapering in at the top). It’s a small bag but can hold most essentials.
I hope you get a chance to try it