No easy fix that I can think of as we've always had to dig them out removing as much as possible and cutting roots with a saw or ax. A pick ax also is useful in the digging. The process is a lot of work & an amazing amount of depth to be dug to get the bulky parts out, but once you're done you'll be pleased with the soil loosened by the roots.

Sometimes, the stumps sprout again like with maples & digging them out is the only option.

You could also cut the stumps closer to the ground & use as pedestals for pots or garden ornaments. The roots take a long, long time to decompose & might cause frustration when digging & planting.

In this past year we've dug out a douglas fir 12" diameter, left 2 cherry tree stumps, & are working on a huge maple stump. The maple stump is surrounded by a 6' diameter pit & we're still not close to getting the bulk of it out as it grows at an angle & is still 2' diameter.

Good luck!

Equipment rentals will sometimes have small stump grinders that are not much larger than a large push mower. Might be worth a try.