The animal is a flightless fowl that weighs around 80kg. It is an omnivore, though about 3/4 of its diet is made up of fruit, tubers, and other plant matter, with the remaining quarter being vermin like rats. The animal regularly fights and is extremely strong, about as strong as an animal its size could be.
What would this animal's meat be likely to be like, in terms of things like flavour and texture?
Having done considerably more reading it will actually depend a great deal on the bird's digestive adaptations. Most mammalian omnivores, like wild pigs and bears, have relatively recent herbivorous ancestors and a distinct "tang" to their flesh due to their relative inability to process the additional lactic and uric acids in their diets due to their consumption of animal matter, particularly stress hormone soaked muscle tissue after a chase.
Assuming they're not particularly well adapted to their relatively recent (on evolutionary timescales) change in diet these birds will be the same. The meat should still have a texture like a wild game bird such as Partridge or Ptarmigan, rather than that of the white meat of a domestic fowl but with a stronger, possibly slightly sour, taste. You might look for historical accounts from those who ate Madagascan Elephant Bird, they were a lot bigger but isotope studies suggest that they had a primarily fruit diet with various additions that their modern relatives don't tend to partake in.
Cassowary, matches your description, tastes (to me) like a gamey chicken/turkey.
The muscles myofibrils convey some taste difference but more texture, hence the gamey 'sensation' since the 'combat' legs are holding up far more weight. Some descriptions I have read tend to use words like 'iron' flavour. The meat is tougher, but depending how skilled you are at cooking, it can be between a lump of rock/ hard leather, or pulled pork.
Tougher than partridge, or turkey on average. Flavour, as per the comments above is subjective, I didn't like it, I felt it to be like deer meat, but dryer like overcooked turkey without proper basting.
Just because 2 animals occupy the same niche and eat similar foods does not make them automatically taste the same. For example, the two most similar animals to what you've described are probably the Emu and the Cassowary, and what they taste like is as different as night and day.
Cassowary is an incredibly tough meat which requires days of slow cooking just to be palatable. There is a fun Australian saying about Cassowary meat that says that you should put a rock in the pot when you boil your Cassowary; you will know that the meat is ready when the rock is done.
In contrast, the Emu is also a large, strong, omnivorous flightless bird but it has a tender meat that is often compared to filet mignon. In fact, most people can not tell the difference between Emu and Beef. Like beef, emu is typically seared and served between rare and medium rare.
When animals are domesticated for food, they almost always taste better than their wild counterparts. With a domestic animal, it tends to get less exercise and get breed for docility and greater body mass making its meat softer than its wild game counterparts. You also control what foods it is able to eat. For example, beef tastes very different based on what you feed the cow; so, we get different flavors of meat like corn beef, grass-fed, grain-fed, etc. In this way, even if your animal is typically a bit gamey those unpleasant tastes can be breed and fed away.
Slow cooking any meat will destroy nearly all of its natural flavors; so, even if your meat is really tough and has a strong gamey flavor, boiling will eventually make it soft and flavorless. Slow cooked pork, beef, and chicken are all practically flavorless and get very similar final textures if simply boiled in water until done. The only noticeable difference between these meats is how we typically choose to season them when they are done cooking, but if you throw a chicken mix into pulled pork, it will taste just like pulled chicken.
It is only when meat is eaten rare and unseasoned that there is a big difference between one terrapod and another.
So, even if you do go with a not so pleasant natural flavor and texture, there are ways to make it taste more or less however you want.
