Because they've been mentioned, I have a Rottweiler and I rarely reccomend them because mine isn't like cassie's...lazy is absolutely not a word I ever use to describe him, lol.
But seeing as it's not a first dog and you mentioned agility.
They're not eager to please as such* (a bit stubborn and independent) but, hugely food motivated and love anything that's self rewarding with an adrenaline rush like agility. They're clever too, mine can pick up a new command in a day.
- though I will add that like GSDs if you build up a good bond with them they are eager to please that person.
No huge shedding, in fact no real coat issues, minimal grooming and pretty Teflon coated, lol.
They're incredibly affectionate and playful, but that's where I get to what can be downsides, they're clumsy, have faulty brakes and are absolutely convinced that they are in fact tiny lap dogs.
Mine (as many do) also has a pretty high prey drive, he's fine with my cats, strange cats who run... I wouldn't trust him with and I honestly don't know whether chickens even if socialised with them from a young age might be a bit much temptation for him.
They don't take kindly to harsh training methods (not that I'm saying you'd use them) but they will also take advantage if you're not great at setting boundaries.
Basically they're all the intelligence and drive of GSDs with the silliness and enthusiasm of boxers...great fun, but you will spend a lot of time working on impulse control.