The Roman diet tended toward more fresh fruit and veggies, plus wine and olive oil. Urban Romans (other than the posh) often didn’t have kitchens. If they had a courtyard (even a shared one) they’d cook meats as well as onions and such outside. Hot food was also sold at take-out joints found throughout Roman cities.
Dunno regarding Tacitus’ meat army. Could have been the monotony of it, our could have been due to the impact on their, uh, eliminations.
Roman armies ate a lot of bread, and more meat (which was usually quite available) than most other Romans, often in stews, which like broths went well with bread.
I forgot to mention that Tacitus said that particular army ate meat at that time. It was a footnote by the publishers that said they preferred bread, so not actually Tacitus.