I agree, a Remove All button in CAS would be super helpful, convenient, and time-saving.
As a workaround, I have saved Sims in my library that act as a blank canvas with everything removed. I title them "START Human M", "START Human F", "START Vampire M", "START Spellcaster F", etc. It's exhausting. When I search "START" in my library, it pulls up all the Sims that I need to begin Creating-A-Sim.
And since we cannot change human Sims into Occults with a convenient click of a button, I have to make a separate Masculine and Feminine for every Occult.
AND because Child, Toddler, Infant ages have their own separate clothing assets, whenever I age down my blank adult Starter Sims, they are auto-dressed and I have to remove each of their outfits and accessories AGAIN - OR - I go through the process of saving another round of "START Human Toddler M", "START Mermaid Infant F", "START Werewolf Child M", etc.
To math:
4 M → Ages (Masculine): "START Teen-Elder M" + "START Child M" + "START Toddler M" + "START Infant M"
+ 4 F → (Feminine)
x 6 Life States → Human + Alien + Mermaid + Vampire + Spellcaster + Werewolf
= 48 separate "START Sims" in my saved library, ugh. Or I spend time on each new Sim in CAS, removing clothing and accessories from 8 outfit categories, to start creating my Sim from a blank canvas.
A Remove All button in CAS would save so much time, trouble and library space.