You did right with the mirror shot. I post this mostly on Garmin forums, but added Ultra:
When sizing a watch don’t turn the Watch toward your face to judge. This rotates the radius/ulna bone position, making the wrist more square and narrow. Instead, look in the mirror with palm toward belly. This keeps the wrist in its natural rectangle and wider state.
Here for example are the 47mm Garmin F6 Ti sapphire and 51mm Tactix Delta as well as 47mm Epix 2 and 51mm 7X solar sapphire on my wrist, and even the 49mm Apple Watch Ultra, both turned toward face and in the mirror. My wrists are just 172mm, 6.75”, and I prefer the larger models for screen and battery.
Note in the mirror view the watches don’t look as huge.
https://www.flickr.com/gp/154102934@N07/M0F602