R6RS was the largest Scheme spec ever, and because of it the next one was actually reduced into two: R7RS-small (core-language) and R7RS-big (extended).
The R7RS-small standard [0] has already been ratified. It is a similar size to R6RS as a document size, but the semantics are about 8 pages, and include a bunch of macros that you don't need to implement, but just copy and paste into your implementation.
Also compare https://people.csail.mit.edu/jaffer/r5rs_9.html and e.g. https://docs.python.org/3/reference/grammar.html which I would consider a relatively syntactically rich language.