This solution is for the hardest part of the "Gold Miner" achievement, which is the "I'm Done" challenge:
I have boosted tons of credits with a dummy account (*see below for how*), which I then used to buy hundreds of geotags, which in turn boosted me to level 15! (I boosted Alex [for race events], Psymon and Mac [for trick events]). I bought the absolute best gear and have loads of money for as many mods as I wish. This makes everything so much easier, just short of fool-proof.
In total I must have spent 200 million credits on the gear, the ranking-up charges (past level 10) and the mods.
*The way you can get unlimited credits is as follows:
1. You need 2 accounts! (easy if you're playing on Xbox One, because of gold sharing)
2. Use either one of your accounts (primary or dummy) and go to "Global".
3. Create an online event (not the 1-2-3 events though!). Patagonia has slopes with very convenient cliffs.
4. Set it up as:
--- survival
--- 10min
--- select highest entrance fee that both your accounts can afford
--- leave the prize money distribution at its default. (this way one of your accounts will get his entrance fee back in full and the winning account gets around 160% of his entrance fee as a prize.) I know the math is weird, but that's how much money you'll get
5. Use one of the accounts and jump off the first cliff you can find. Use the other account and barely beat that distance, then also jump off a cliff. It doesn't matter which account you win with first, since you will be alternating the winner every race
6. Wait until the event is over in 10min and collect your prize money.
7. Now you switch the account that wins the next race. This way, both accounts will receive the 160% prize money every other round and the money for both accounts will increase.
8. Since both accounts will have more money over time, that allows you to select higher entrance fees and will therefore result in higher prize money. Thus, your money will increase almost exponentially and soon you will have prizes around 100 million and more each race.
Hope this helps and anyone is still in need of a solution for this ancient game.