In my case it is a dog. Initially I used a dilute hydrogen peroxide and water mix, but a few months ago I began using a chicken flavored enzymatic toothpaste for dogs put out by Virbac (CET). I was noticing his gums were showing just the very early stages of an issue.
He also gets one Dentastix (sp?) a day.
As for the brushing, I just do my best. Some days I can get the brush in there where it needs to go and work on the areas of concern. Other days it's not so easy and he turns his head or tries to chew the toothbrush.
The main thing for me is to part his lips and mouth and keep it open and in a relatively stable position long enough to get the brush between his teeth and his cheek so I can spend a few seconds or longer really brushing. He likes the taste of the toothpaste and that usually distracts him long enough that I can brush. It seems harder to brush his very front teeth, but they don't need as much brushing as his back teeth and gums.
He really needs a good cleaning but I've been reluctant to take him in since the pandemic began.