I bought a Grom about 3 months ago - my first bike.
I love it, except for the mirrors. I drive a lot on one lane roads and not being able to get a quick glance before turning my head is driving me nuts.
I'm guessing bar end would be best but trying to find good reviews (that is, not LLM BS or not so subtle marketing disguised as blogs, etc) has been challenging.
I didn't have any recommendations per se, but one thing I think would be very useful and functional are tall mirrors stems.
I ride a Harley with standard mirrors above the handles, and I always have to take me eyes off the road to glance at what is behind me. But if my mirrors were like 3 or 4 inches taller, I think they'd be in my natural field of vision.
My suggestion would be to try and find some taller ones.
The stock Grom mirrors might be tall enough but they're close to the middle of the bike. Maybe just throwing on a wider set of handlebars sooner rather than later might help fix it then.