In order to get this achievement and all other "Earn over --- newbucks" achievements, there's a simple strategy to complete this quickly and pretty passively.
1. Make two corrals with the high wall, air net, auto feeder, and plort collector upgrades.
2. Fill both corrals with 10-15 largos. Preferably late game slimes like quantum/crystal and
3. Use their favorite foods to gather twice as many plorts. All favorite foods can be found in
the slimepedia. The favorite food of either half of the largo will give twice the plorts for both
halves of the largo.
4. Harvest to your hearts content! You can complete this process quicker by sleeping until the
next morning, refilling the auto feeders, emptying the plort collectors, and going right back
Here's my video for every achievement if you'd prefer. Skip to 4:27 for Well-Off Rancher.