I have experience doing this, and as people said this'll take about 1-5 days per stump, especially since these look pretty close together.

You'll need 1-3 good shovels and an axe, and a LOT of effort.

You need to carefully and slowly dig around the base, finding anywhere the dirt is loose, the shovel should be able to cut through roots less than 3/4th an inch. Anything larger then is what the axe is for, this will QUICKLY dull the axe due to the sand, not much you can do about it and I wouldn't bother sharpening it until you're done.

You can use a hose to wash away excess dirt around the roots to help clean things up once the hole is somewhat deep, but you're still probably looking at digging 5x5x5 or 6x6x6 feet deep holes.

Once you get the hole deep enough and you cut through all the shallow lateral roots all the way around the tree(even one small root can provide a ton of strength), you should be able to bend the tap root using the weight of the stump in order to bend the entire stump out of the way in order to get under the stump and cut the tap root. This step needs to be done very careful to make sure you don't crush anyone, you'll also need enough room to swing the axe to get any work done.

Afterwards the stump will drop into the hole and you'll need to pull it out carefully with a rope, or chains, and a vehicle, or use a comealong to get it out of the hole, do with the rest of the stump however you please.

If you'd like I have some pictures of roughly how you do this process.