Not sure where you are in your “progression” but you can refer to the chart in this link to see which sidequests give which Crystals during Class playthroughs:
If you have only worked on one class then you have 4 additional classes that can potentially generate Crystals via playthroughs. Although additional playthroughs might be repetitive it’s also a good way to get additional resources and opportunities to develop the additional classes for varied playability.