You don't knead soda bread for long, some methods call for no kneading at all. One reason is texture, soda bread should be a bit crumbly, stretchy isn't what you are aiming for. The other reason is to get the most out of your leavening agent. Baking soda and buttermilk (or any other acid) will start to react immediately on contact, although the reaction is much slower at room temperature than oven temperature, so the longer your wait the less rise you'll get.
I like to knead my soda bread for about 60 seconds, just for a bit more structure as I like it that way, but that's as far as I go. I do it all by hand, a mixer is overkill in my opinion as you'll spend more time cleaning up than you will actually mixing anything. Others may have a different view on that, there's no right or wrong answer to that one.
You can't add the soda later in the process because you won't get an even distribution, it needs to be mixed in the flour at the beginning.