That's really a fine line , high volume , high liquidity and comfortable trading spread , all really appropriate for scalping that brings profit in a short time.
Scalping as short-term trading with small target 5-10 or max target 20, I think ned to use balance risk-reward ratio at least 1;1, sometimes trader use larger stop loss than the target, once loss may vanish all previous profit.
Totally do-able, 1Bln / 20 days in a month is 50Mln per day. Using 10x leverage it can be done with a 5Mln $ account. With good money management one could position that amount across multiple pairs with a combined Total Position Value Stop loss of about 0.33% x10 leverage that would be a stop of 3.3% of 5Mln account balance per day.
....but if what you say is true, you have way more than 5Mln and are not repositioning it every 24hrs, that is too much work. You could be using an EA, but that is just dumb. So that leaves you to weekly positioning with at least a 25Mln account. If you really have that kind of money, you should hire me.
Oops...typo error! This is what I meant. The best pairs for me are EURUSD and GBPUSD because they have high liquidity and less volatility.