I've worn bifocals for decades (I'm 56) and am both astigmatic and near-sighted. I have tried a couple variations of helmet/eyeglass-mounted mirrors with circular and rectangular mirrors of various sizes and long & short mounting arms. I can use such mirrors but they constantly aggravate me.

There are a few factors that add up to outweigh any/all positives for me.
1. The mirror always blocks something in my field of view no matter where I look.
2. However "safe" anyone argues a helmet/eyeglass-mounted mirror is, its very proximity to my face/eye makes me mentally uncomfortable in terms of potential damage to my face/eye in an accident.
3. My right eye is my stronger eye, but due to traffic patterns in the US, I pretty much have to mount the mirror on the left. This causes me noticeable, but not debilitating, eye strain over long rides or even medium length rides in high-traffic situations depending on how much mirror use is required.
4. I have two slightly different pairs of bifocals; their frame and lens sizes being different, each requires a slightly different physical location for best mirror imaging and conscious, deliberate eye focus. Ultimately, the mirror just doesn't sit in a natural location for me to focus on it with either pair of glasses.

In addition to all of the above and probably most significantly, I have to close my right eye to actually focus on the mirror's image even for a second. Having spoken with numerous people using similar mirrors, this is apparently not that unusual. But others have overcome similar initial difficulties (ex., military helo pilots are taught to simultaneously focus one eye on a video screen at eyeglass distance and the other eye at far distance). After months and over 4000 miles of use, I haven't been able to do so.

Ultimately, I'm simply not comfortable with a helmet/eyeglass-mounted mirror and have gone back to a bicycle-mounted mirror. YMMV. Good luck.