Levels scale up non-linearly, though, so you could pretty easily fit Phantom, Doc, and Courtney into the lower end of 1-5, Daredevil the middle, and Batman/Cap at the 5, maybe 6th level. 6th is the point where characters become capable of literally superhuman feats of otherwise mundane athletics, so I guess at best there's a fuzzy area of roughly a level's worth in between each 'bracket'. You had mentioned the ability of the GM to keep characters relying on teamwork/guile/luck rather than their sheer uberness, but I'd consider that a measure of the opponents they face as much as their personal ability...Batman fights Joker, Beowulf fights Grendel, Superman fights Doomsday and Darkseid; they get matched to enemies who can still challenge them even with the power they've obtained (mechanically, higher CR bad guys).
11th level+ is still a big bracket in itself for that matter...people like the X-men would sit at the lower end around 11th-12th, the cosmic heavy hitters like Supes and Green Lantern at the 17-20 range. So maybe split it into 1-5 'peak human', 6-10 'mythological', 11-15 'planetary legendary', 16-20+ 'cosmic legendary'. Heck, even Superman still goes on 'dungeon crawls' once in a while, though where Batman's idea of a dungeon is the sewer tunnels beneath Gotham, Superman's 'dungeon' is Darkseid's Palace on Apokolips.