Reduce snap!'s features
This is a bit of a frame-challenge, but what I'm understanding from your question is that your goals here are to: 1) Not overwhelm your students and avoid them getting lost in a plethora of blocks they won't need, 2) Force students to understand how to implement more complex blocks themselves.
Have you considered instead of providing a restricted version of the language (which, could be complex to install and update and ensure every student is using the same version of), simply providing a list of &quot;recommended&quot; blocks? This could help to achieve your first goal, to act to guide students as to what they'll need for an assignment and avoid them being overwhelmed. And, this still allows students who want to explore a larger set of the languages features to do so if they want, something which I think should be encouraged.
As for your second goal, this depends on your class but it may be effective to simply create an assignment around implementing more complex built-in blocks and enforce the restriction specifically for that assignment.