The problem with ancient recipes is that everyone who recorded one presumed the reader had solid knowledge of how to use ingredients and cook the dishes. So many ancient Roman recipes sound horrid, because there's no distinction between things intended to be used as a pinch and things to be used freely. Anyone who has ever spoiled a recipe with a couple of extra shakes from a fish sauce bottle will understand. That, by the way is a very credible for garum, fish sauce from Italy or from Asia. There are very few things that can't be found today. One herb that I know of that was so popular then it's long extinct now. No one knows what it tasted like.
Meat was not so common at all. Fish was more highly thought of. The picky even specified that a certain fish must be caught just below a particular bridge on the Tiber. Pork was common, frequently in sausages. (Sausage presents sellers with perfect opportunities to use diseased animals and perhaps kitty.) Game was prized, as it was in upper classes society for hundreds of years beyond. There were some, to us, peculiar willingness to eat some fowl that we would never consider, but that's true of the Medieval period also.
Beef, when it could be had, tended to be very tough, since the common source was old draught animals that were no longer thrifty workers. Boiling was about all that could be done with it.
Garden vegetables were popular. Until modern times, anyone who could kept a home vegetable garden. Cabbage seems to have been very popular. But like later time, legumes were king of common foods. Peas, beans, etc. Wheat was essential and was imported largely from North Africa. There was a bread dole ("bread and circuses"), and riots when the ships from Africa were delayed. As in many times, a bread diet wrecked teeth from the grit of grinding stone. Roman troops in some periods carried parched, ground wheat that they could mix with a little water and eat on the march. They were also habitual eaters of that Boy Scout favorite, simple dough snakes wrapped around an iron rod over the fire. And like other pre-modern times, white bread was preferred, and dark bread left to the lower end of society. Bean flour bread to the lowest. Roman breads were flat loaves. Leavening was not routine.
Remember they had none of the foods that later came from the New World or the Far East. No peppers. No potato. No tomato. No turkey. No maize. They didn't use churned butter. In fact, the "barbarian" Germanics were referred to by one writer as "our butter-eating friends." Beer was also associated with barbarians. They had no sugar, although they could get honey and could use verjus and wine must. They were given to heavily scented and spiced wines. Of course many had to do with sour old wine. But again, that was true in Medieval England.
They did like fruits, of which the European Medievals were suspicious and avoided them raw, like they cooked greens to much for fear that they were unhealthy.
Mice and songbirds were indeed eaten, but more a novelties at high class to-dos.
The average man ate lots of wheat porridge, with beans and vegetables, if he had them. Not inspiring stuff. We know quite a lot about what was eaten by the remains of food in places that were destroyed by fire or volcanic eruption. Even old cooking pots can be analyzed for their past contents.
But we just can't know their tastes, what flavor profiles they likes. It is clear some would seem bizarre today. We know, for instance, that they ate pates, because we find the molds. But we don't know how they made them. So anything that purports to be a "Roman cookbook" in the sense of today's cookbooks are always someone's guesses of how ingredients were used and must inevitably be distorted by today's tastes.
And Romans had many popular tricks to make one food seem to be another, for various reasons. The habit survived into modern times in Europe where the "rabbit" might be "roof hare." Meow.
One gets the impression that, like Medieval peoples, the ancient Romans liked a lot of very strong flavors and threw caution to the wind when seasoning. That is, if one could afford it.
A typical Apicus recipe:
Sauce for boiled boar:
PEPPER, LOVAGE, CUMIN, DILL SEED, THYME, ORIGANY, LITTLE SILPHIUM, RATHER MORE MUSTARD SEED, ADD PURE WINE, SOME GREEN HERBS, A LITTLE ONION, CRUSHED NUTS FROM THE PONTUS, OR ALMONDS, DATES, HONEY, VINEGAR, SOME MORE PURE WINE, COLOR WITH REDUCED MUST [and add] BROTH AND OIL
Silpheum was that now unknown plant that was too popular for its own good. It was said that Asafoetida was used as a substitute, but that's not too helpful, since we don't know why. But consider the recipe above. No quantities. Was it powerful and sickening. Or subtle. Apicus doesn't say. But boiled boar probably needed a lot of help.
So get a good Apicus, like this one translated and annotated by a chef, and see what you come up with that you would eat if you were a Roman.
https://www.amazon.com/Cookery-Dining-Imperial-Rome-Apicius/dp/0486235637/
For garum, try, Colatura di Alici, whch is likely very close due to continuous tradition. Surely it matches some garum, because there were many branded versions in ancient Rome.