To make vegan: I think you might be able to use flax eggs as a replacement, but I haven't tried it myself yet.
Feel free to use any nut butter instead of cashew butter. If you want to make it nut free, try tahini or sunflower seed butter. Sunflower seed butter will work, but your muffins may turn green; sunflower seeds contain chlorophyll which reacts with the baking soda/powder in recipes when heated and then once cooled, can turn green.
Bars are freezer friendly. If you want to freeze them, simply wait until they are completely cool, then store in an airtight container or freezer safe plastic baggie.