What is the opinion of Reddit about the
Wahl- Beard Trimmer, Battery Operated Groomsman, Black 9906-708?

I think adding yet another “have to plug it in” thing is annoying, so I’d recommend this AA powered one:

https://www.amazon.co.uk/Wahl-9906-2017-Groomsman-Battery-Moustache/dp/B00014TDLC/ref=sr_1_4_a_it?ie=UTF8&qid=1501796763&sr=8-4&keywords=electric+trimmer

Battery lasts for a very, very long time.

I used AA powered ones, and have a charger (as I have other AA/AAA devices)

But, if you wanted to make it fully USB powered, there’s Lithium AA rechargables that can be recharged with an integrated USB port.

Wahl makes reliable trimmers: https://www.amazon.com/Trimmer-Battery-Operated-Groomsman-9906-708/dp/B00014TDLC/

I don’t own, but like this: https://www.amazon.com/USB-Rechargeable-Lithium-Batteries-ECO-Friendly/dp/B079KCTSNP/