The type of coffee you’ve described is my exact preference for espresso/flat-white. I’m not a world-class barista by any stretch of the imagination but I’ve made a lot of med-dark espresso.
I also don’t have a huge amount of experience with comparing grinders as all but one of my espresso grinders have been flat burr. But to me, Mazzer (58mm and 64mm) just does such a great job of that style of coffee that I’ve come back to a Mazzer twice after trying other grinders.
The Super Jolly I just got, despite being a pretty dated grinder these days, consistently produces thick, rich, chocolatey espresso over and over.
For brew, I’d always choose a conical but for espresso, a big ol’ no-frills flat-burr will always be my choice.
I’m also still not completely convinced that single-dosing for this type of roast has the noticeable benefits it does for lighter roasts vs on-demand.
Before the pitchforks get thrown, I’m not saying there’s no difference, there obviously is, but dark roasts seem more forgiving to me. When single-dosing isn’t a factor in choosing a grinder it opens up a world of possibilities. VA Mythos, Compak, Mahlkönig etc.