what are some ways sd garchomp can be used without being a downgrade to dragonite, kyurem or great tusk on HO
Garchomp has a surprising mix of raw power and speed that the others can't match. I'm not a fan of SD Scale Shot sets because that move is ass and Chomp isn't that great of a sweeper. It's better fitted to serve as a breaker imo. Life Orb sets can force big damage on Tusk, Lando, and Gliscor and threaten Corv and Mola with the help of SD. Rocky Helmet alternatively serves as a trade mon that takes down physical attackers, while it is free to run hazards if it wants. This isn't an endorsement of TankChomp btw; I think max attack and speed is the way to go without facing competition from other Ground types.
As for why you would use Chomp over the others, Chomp has a great speed tier for breaking (can afford Adamant as well) with it out speeding Kyurem and fast Zapdos while not compromising any power. It's rock resistant unlike Dragonite and Kyurem, so it has more item freedom and requires less team support. As a hazard setter, it beats all but Corv and Ace, unlike Ting Lu which must trade with hazard removers, Tusk which often would like to run other moves and doesn't really like opposing Tusk, and Lando which really only beats Corv and Hatt.
Chomp does have its flaws however. It is unfortunately much slower than other offense staples like Oger, Rai, Zama, Val, and Pult, all of which threaten big damage. It's also a Ground type that cannot switch into Bolt, a responsibility often left to Grounds on HO. And maybe the biggest flaw is that it stacks weaknesses and overlaps with several offense staples. Garchomp locks you out of Lando, Tusk, and Ting Lu, mons with much more utility for HO teams, as well as introducing another Fairy and Ice weakness to the team, making it difficult to pair with other Dragons, Darks, and Fighting types.