Q. Can I create a Subject Readings list for myself or to share that isn't for a teaching site?

Yes. You can create a reading list for yourself that isn't associated to any Moodle site. Keep in mind that others can still view it via FIND LISTS if it is published. You can also create a list that's associated to a project (PROJ) or training (TRNG) Moodle site, it doesn't have to be a subject Moodle site.