@K-UwU did read what you posted. Yes weeklies and other forms of task rewards do inject massive amount of meseta into the system, much like when ARKS mission was introduced in EP6 on the JP server - it being so easy to do clearly had effect on the economy. And this was per character so people that bought 20 characters earned alot more than freemium players. Before that it was just running the weekly time attack client orders (TACOs) which also gave over 1 million meseta for completing them all. Again, having multiple accounts is NOT against the ToS and people that can run multiple clients at the same time would bring alts with them as they do it on every single of their characters. Only real way to solve the problem is to remove all the easy money making mechanisms in the game but I can't imagine the free players would be fine with that.
There's very few ways you can prevent players having multiple accounts especially on a free game, even in subscription games - people are willing to pay multiple subs for multiple accounts.