Teaching experience is worth almost nothing in the academic job market (compared to research outputs). I now tell new PhD students to avoid casual teaching. There will be plenty of time to “get teaching experience” after you submit your PhD (and you’re trying to get your first aca job). Make full-time staff do the teaching and assessing that’s in their contracts. If full-time staff have too much teaching they should take it up with the union or the management of the department.