My strategy is to use existing solutions as much as possible. So I am using the SEBLOD way and so far I am very happy with it. (I also appreciate the seblod changelog, when using their functions :)
For SEBLOD AJAX URL I use following params:
file=( URL to your PHP script )
Since SEBLOD 3.16.0 ajax task requires scripts to be allowed by extensions. The URL must contain a parameter referer which value refers to the Joomla extension(component, plugin, template ) that allows your ajax task. For example:
The selected extension installation XML file must contain the tag cck_ajax with the URL to the target php file (identical to the value of the URL parameter file) as a reference e.g.
I don't think it is necessary to develop a special extension in order to exploit SEBLOD ajax functionality. The file is just a PHP file. On the other hand, yes since SEBLOD 3.16.0 the PHP file must be checked in a Joomla extension XML.
If you look at the method checkAjaxScript of JCckDevHelper class
You'll find all the options where you can put the file reference. You can add the reference code to a templateDetails.xml of your custom template, for example.