I have a lot of brush stumps around my home. I am working on the first one. It is too hard. It is deep rooted and tangled.
Some videos show that they use a truck to pull them out; but it is not an option to me.
You could try to remove that brush stump by using farm jack and chain. It helps to make wooden support to stabilize jack, attach one end of chain to jack and other end of chain wrap around stump. Start pulling and if necessary cut bigger roots to make it easier to remove. After removing stump dig and remove smaller roots left in the ground.
IF you kill the stump (keep applying brushwood killer till there are no more leaves!) and then leave it for 2 or 3 years, it will decompose underground naturally. That is slow, but the "minimum effort" answer.
If you wanted to pull the stumps out with a truck, you should have left 5 or 6 feet of trunk above ground, so you have something to fix the chain around and you can get plenty of leverage on the roots. But you say that isn't an option anyway.
Another "mechanical" method is to hire a stump grinder. You may want to a tree specialist to do the job for you - operating a grinder is quite hard work because the effort of swinging it from side to side to chew your way through the stump isn't power-assisted.
Trying to burn out stumps that are not completely dead and bone dry is usually a waste of time, especially on small stumps with many "trunks" like yours. It works better if you felled a tree with a single three-foot diameter trunk, but it's not as simple as just "pour on some lighter fluid and watch it burn".
When you get rid of the stump, don't forget that the ground will still be full of thick tree roots. It will be hard to do any deep cultivation until they have decomposed enough so you can chop through them with a spade - which will probably take a couple of years. If you just want to make the area into a lawn, the roots won't be a problem.