I actually really like this idea. The organ gun was definitely more widely by a lot of civilizations, so I think seeing it become a regional unit would be really cool. Especially since I already think it’s a bit odd that the Portuguese even get it as a unique unit, since there’s not really any archeological evidence they ever even used them to my knowledge.
Personally, I think it’d be cool to see the portuguese get the Berço, which is a breech-loading swivel gun they used on ships and fortifications in their naval empire. It’s not purely unique to them, but it definitely makes much more sense as a unique unit than the organ gun. As you said, I think it’d make the most sense if it was a heavy hand cannoneer unit that has a faster rate of fire.