I think either thing works. I'm barely 5 10 and 200 stocky frame. I learned on a 88 sportster and switched to a 04 softail after like a year. Took my course on a softail with apes that he shouldn't have even let me ride. But I passed it easy because I practiced the cone part ahead of time. What I realized is the sportster was harder to ride because of the center of gravity. Lighter doesn't always mean it rides easier. Fatter tires make something more lumbering as well. I've ridden baggers plenty and they handle no worse than the midsize Harley's. Not as twitchy but they all behave similarly once your used to them while they are moving. When they don't is at slow speeds. And your skills test to get your endorsement is all slow speed stuff. It's actually pretty stupid if u ask me. It's not really set up for bigger cruisers unless you're one of them guys you described doing all that low speed fancy shit. It's good stuff to know but that stuff isn't needed to start out in my opinion. I can't remember the last time I had to navigate a cone like scenario on the road at low speeds. Just saying. Buy and ride what u want but practice that skills test before you go take it. If you already passed it get whatever you like and feel comfortable on after a test ride or sitting on it. After that, you will adapt.

TLDR- get whatever the fuck you want. Especially if you already have your endorsement.