If you are ahead on the conquest game, you might be able to crush one of your own colonies by letting it go and declaring on it. From there, you could either release a country that still has cores, or use client states.
Client states use the same idea groups you have these days, so you can also use them. Simplest would be to move your cap to new world so you can immediately make them there, rather than doing transfer occupation shenanigans. Bermuda should suffice, you can then make them in NA and give them land in SA too without them making colonial nations.
Note that in OP's case, this isn't necessary (as OP discovered). As long as you average 6 conversions per year, you can directly convert 283 provinces in 47.167 years, so even such a relatively paltry rate (by EU 4 standards) would get the job done. For religions like Christianity/Islam, 1 province per year per missionary on average is trivial, and significantly faster is possible.