Not to dissuade you from making the recipe you found but the best imperial stout I ever made was the original RIS published by Ron Pattinson. This is my version of that recipe adjusted for my equipment profile. https://beersmithrecipes.com/viewrecipe/2581876/1848-barclay-perkins-ibst
The second best imperial stout I ever made was the Black is Beautiful IS published by the Homebrewers Association about a year ago... https://www.homebrewersassociation.org/homebrew-recipe/black-is-beautiful-homebrew-recipe/
i have considered that ron pattinson 19th century style black/brown/amber malt imp stout, but im aiming for a highly hopped US style one that has intense malt flavours going on and also a lot of hop flavour/aroma.
im thinking there are four concepts i could aim at. feel free to correct me or suggest more.
1. "us bourbon barrel style" variety of heavy roasted malts with moderate supporting crystal. high hopping/ibu from american hops. high ABV and oak/whiskey potentially
2. ron pattinson's black/brown/amber mid 1800s imp stout. people who make it swear by it.
3. "very dark black beer" with a ton of layers of less-harsh roasted malts (dehusked carafa/midnight wheat,etc) and very dark crystal malts, maybe less IBU even.
4. "double stout", a dry irish stout but with the ingredients mostly doubled, a really strong normal stout with twice the ABV or more but maybe 1.5x the hops and roasted malt.
i have traumatic experiences about beers with two high an FG/too much crystal so can't do 3. im leaning towards 1 or 4.
also was that 1845 style imp stout really made with 210 IBU?