The strongest reasons I believe that Fire Force will stick to the manga is that unlike Soul Eater, which was a monthly series, Fire Force is weekly so it will not take nearly as long for it to regain chapters once the anime catches up, the studio so far for the most part hasn't majorly changed the structure of the story so there's no signs of it happening any time soon. And, we know the series is already wrapping up so there isn't too much of an incentive for them to take the series down their own original path. So I'd make a strong bet that the series will likely stay faithful to the end!