It might be an optimal choice to do both; give an option to hand craft absolutely everything down to the parts of the cells and an option to have some basic premade vague features (or more defined things) that you can further edit. Even if their aren’t even basic guiding outlines, a very, very detailed tutorial will be needed for a system that can be crafted down to the cell like that.
I saw an idea like your on the old thrive community wiki, they moved sites, and I personally liked that idea. A premade ‘part’ (don’t get annoyed at someone calling them parts again) that you can slap on or edit, if your feeling adventurous you could even make your own.
One problem with a system involving distinct parts is that in order to simulate gradual undirected evolution, the parts would have to be extremely simple and broad, to the point where the player may as well be hand-crafting any complex bodypart
Easy, become a plant and eat the Sun
If you’re way bigger than the planet’s life, eat the planet itself! (jk dont do that)
“MMmmmmm tasty Iron, wait why is it hotter down here? Do I smell iron as a liquid? Let’s drink it the-”
Sveve the Terravore