The Pracinha aren't known for the level of their equipment or training, they became notorious for their elan and adaptability. They went to successfully conquer highly fortified positions and, despite being from tropical lands with little thermal variation, were suffering less from European winter than their northern allies. Some foreign generals even went to say that, to learn how to conquer a city, watch the Brazilians, referring to the Pracinha.
Compared to other UUs out there, the Pracinha is closer to something like the Impi and the Panzer. The former two weren't known for having superior equipment to their contemporaries, quite the contrary, yet surpassed all expectations with their elan or doctrine.
Right now, their position in Brazil's toolkit makes sense, as Radio is about the time in which most AIs took an ideology and new enemies are formed. The Pracinha fits the CV focus of Brazil and is very easy to use for a non-warmonger civ; just mass purchase them with Draft Registration and the gold from the UA and send them to the enemy, no matter how far the enemy is due to their bonus movement and Amphibious promotion. Even better if the enemy is far away, due to the bonus

CS from the Expeditionary Force promotion being based on distance from your Capital.
The timing around Radio is also very convenient due to how important the technology is for Brazil. The technology has an empire-wide buff to tourism modifiers, it has the last yield buff to Brazilwood Camp, it unlocks the Broadcast Tower for major tourism gains and, if Brazil went for Industry (currently the only Industrial policy tree with a WLTKD related bonus), it also unlocks the Broadway for another powerful theming bonus. Their current placement allows Brazil to wage war without having to focus the war oriented bottom part of the tech tree, focusing instead on the upper part that has the world wonders and the cultural bonuses of the Modern Era.
If the Pracinha needs a buff to account for their Modern Era timing, three thematic ones are movement-based ones (e.g. bonus movement in hills and snow, akin to the one from the Pictish Warrior), city conquest ones, and against fortified enemies.
I like this suggestion, as the Amazonas are from a period of time in which the Brazilian Navy had a clear superiority in the region, plus happening during Pedro's reign. However, there is no theme or synergy between this unit and Brazil's toolkit. Brazil isn't themed around navies or oceans (in fact, they are themed around rainforest instead), and Ironclads aren't timed around anything particularly important for Brazil's kit. There is no clear use for this unit in a non-militaristic civ that is clearly focused around Cultural Victory, and a high risk of ending as another forgettable UU in a civ's kit, much akin to Shoshone's Comanche Rider or the Austria's Hussar.
The unit somewhat resembles the discussion I had before about the Minas Gerais, the UU Brazil gets in civ 6. It is also a naval unit, known for
starting a naval arms race in South America and even starting an
hilarious international conspiracy theory event (same video at 6:23). It is impressive, but still doesn't fit any important role in Brazil's kit or gameplan, and would be a better theme if it were on a mod that implements a more militaristic version of Brazil instead.
For now, I prefer to keep the Pracinha, as they fit a clear purpose and have a great timing for what Brazil wants to do in Modern Era. Yes, they are a late unit, giving them relatively little time to impact the game, but they do affect your plans in Modern Era enough to be a good addition to the civ's toolkit.