Check out this trick! Short version, use your angle grinder on them.
If you took the tree out, you should dig out the roots and backfill. Otherwise, they will decay and you will have a trough in the yard.
Here's another option for your grinder https://smile.amazon.com/gp/product/B07R5K5J38/ref=ewc_pr_img_1?smid=ADBW1YU9749J4&psc=1
Well, I have had very different results, still filling in some trenches.Help us out here, Scott, with a little more of your experience in this field. I had some pine trees taken out in 1996 and I'm still busting blades on the roots 2.5 decades later. What can we do to speed up that process other than what's shown on that great video?