Nah, I think Garoua is safe mate. It's a neighbouring region but not that close. As far as I know, there has never been a real terror threat in Garoua.
Looks like I could be your travel guide here
. Well, travel from Garoua to the other main cities is best done by flight, the distance to the other towns hosting matches makes road travel really very long.
Concerning Covid restrictions, it's like you said, if you're tested negative you would have absolutely no problem entering the country and going around.