This account of how brazil became an independent state is very... strange.
Regardless, for all posterity, the 1825
Treaty of Rio de Janeiro literally opens with this:
Maybe they teach it different in brazil, but elsewhere it's a fairly straightforward story of fighting for independence from a european power.
What's unique about brazilian independence is that you have a transition from a European monarchy to a local monarchy with
the same royal figure on the throne. How often do you hear of something like that? Basically never.
The problem with generating Unique units, of course, is that with the entrance of numerous post colonial nations into the franchise, the UUs all end up clustered in the same few eras (occasion renaissance, but mostly industrial+modern) which means that FXS has to be careful to try to make sure we have unique versions of units that
aren't just riflemen and infantry. That certainly makes things harder, but all "young" nations have this issue. Does the trend of giving the USA unique WW2 airplanes serve us better than some other thing? The Minas Gerais isn't a terrible pick for unique battleship, I think you could maybe pick the actual HMS dreadnought, or maybe the Yamato for japan, instead. Most battleships never saw much action.
That said, I do think Voluntarios de Patria could have some interesting gameplay aspects given the name translates to something like "homeland volunteers." It's also fun because Pedro II created them. Especially if you had some kind of levy mechanic or unit line to work with. (Past games have had levy action, Humankind has a unique Levy/militia unit line too.)
In any case we are talking about a somewhat limited set of units to replace, and while you could make an entirely unique unit like some civs get, I'm not sure a ranger/scout esque thing really fits the gameplay of the time period. (Rangers in civ6 struggle a lot as it is.)