Might seem like a dumb topic, but need input on the quickest way to recoop diamonds.  Can't believe I'm out, but all these changes over the last couple years with gold critters and pottery lottery, I'm down to almost nothing

The only way to do  it is by spotting/attracting wildlife.  Caterpillars are good; you get alerts, so don't have to water too often to spot them, they give quite good diamonds, and people often avoid them because they're "pests".
 
  Setting up your own garden well for something which gives good diamonds can help too, but it's not always easy if you're aiming for a particular task.  But anything you can spot or attract will add to the stash, and it soon mounts up.

(Unfortunately, we can't gift diamonds anymore so wildlife really is the only way :)  )

I tend to have a day or 2 set up for high value things this saves me from using a lot of gold hunting ….. I usually do a topiary gold uni trim and pop out extinct poles this really helps me rebuild my stash x

The extinct wildlife and the pretty worms work well. There are still people hunting the extinct and even the worms give good diamonds. If you can do that for several days then it will help.

Using a birdbath helps, too, as the chicks are high diamond critters.