Presets are built in styles that create a different view or colour scheme. So yes multiple child themes need to be made and each one with it default preset in place,
However it would be a good idea to have Presets as an option now that we are moving to a new way of styling themes, but thereby lays a problem with Moodle, in that some of its CORE code is old and has not caught up with the latest technologies on offer, that's why we only have Themes available for Courses and Categories, etc., and not just a simple Preset which would work much faster and be less hassle. That said, Categories and Course themes can be a real pain as one takes preference over another, if my memory serves me correctly, which in some cases can loose the whole feel of change and end up with your Default theme instead.
The bigger problem now is that Moodle 3.3 will not be updated other than for security issues, whereas 3.4 is just on the borderline for updates and will be dropped as soon as 3,6 makes it's entrance later this year. So if someone were to write a script that would allow Presets to be an option within a course or a category then that new Improvement could be back-ported into 3.4 but not 3.3.
Just a bit of background for this discussion if needed.