Build your own
Pros: it's fun, you'll save a bit of money, you get to pick the exact parts you want
Cons: no tech support, it's time and energy consuming, you might not be able to find the graphics card you want in stock (for a reasonable price)
Prebuilt
Pros: you get tech support, you save time and energy, they have RTX 3070 and 3080 (and RX 6800 / 6800 XT) in stock
Cons: you pay more, you get maybe worse parts, it's not fun
I built my SFF PC not so long ago, you can check the specs out in the sidebar. 3070 is not *that* hard to come by, even if it takes up half your budget.
If you wanna go prebuilt, I'd say Dreamcore Fuel is the best choice, just don't bother with their water cooling options.