> One of the conditionals needs to be built-in.

That's actually not true. IF can be implemented in terms of LAMBDA.

    (defun _true (then else) then)
    
    (defun _false (then else) else)
    
    (defmacro _if (condition then else)
      `(funcall (funcall (function ,condition) (lambda () ,then) (lambda () ,else))))
    
    ? (_if _true (print 123) (print 456))
    
    123 
    123
    ? (_if _false (print 123) (print 456))
    
    456 
    456
    ?