Read the first four, will check out the fifth, it looks interesting! And yes, I really should.
I forgot to recommend you any books! The Wheel of Time fits your criteria perfectly, but it has some flaws.
Riyria is also a great series, it's about two thieves, Royce and Hadrian.
The Vorkosigan Saga, by Lois McMaster Bujold is one of my favourites though they're science fiction and not very long. I started with Shards of Honour, which at first I wasn't terribly impressed by, but read on and by the second book I was deeply in love.
I hear The Black Company and Malazan Book of the Fallen are good, but I haven't had a chance to read them yet, so can't judge.
The Mortal Instruments, by Cassandra Clare is an awesome series. Don't be turned off by the first book's terrible cover, it's one of my favourite series. I wasn't the biggest fan of Jace but then I never like those types. Malec was my first ship
Wild Cards is definitely epic in its scope, though very dark. I wouldn't say grimdark, but it's treading a line. Not quite 500+ pages but still doorstoppers (I think; I have the Kindle editions.)