If you properly want rid then dig it out, which is hard work but quite satisfying, or hire a stump grinder.
Its more the roots that bother me.
@nickjb is right if you want it gone
now. To dig it out I would have left it taller, to allow more leverage when pulling it to-and-fro. A tree can be held in place by surprising few, and small, roots. Once the main ones have been cut the best option is rocking it back-and-forth to snap the rest.
If you just want to stop it regrowing, especially the roots, then you can use a stump killer. AFAIUI the only ones permitted for domestic use are glyphosate based.
Ordinary glyphosate based herbicides are no good, they are (IIRC) c. 10g/L. You can much more concentrated versions such as
https://www.amazon.co.uk/Roundup-Tree-Stump-Weedkiller-250ml/dp/B01BY4FFPK
which are 360g/L. The instructions for that say to water it down to water it down to 36 (or) 72 g/L when using it as a stump killer which seems a bit odd to me.
You can also get special 'capsules'
https://www.pitchcare.com/shop/prof...plug-max-tree-stump-killer-100-plug-pack.html
https://www.frjonesandson.co.uk/products/ecoplug-max-100-pack/#
of glyphosate at an even higher concentration. You drill a 13mm hole of the right depth, put the capsule in and tap it down. This releases the glyphosate and seals the hole.
Those links are for 100 capsules. I have seen people (mostly on Ebay I think) selling smaller quantities.
Using glyphosate will kill the stump & roots, it will not remove the stump. Over time, i.e. a number of years, the roots will rot and then it will be much easier to remove the stump. I killed some ash(?) trees that were 4-5" in diameter and after 7 years the roots were
completely gone - I leant on one stump and it gave way, the others I rocked back-and-forth by hand and they came out.