I actually think having at least two independent implementations is important to making sure the spec is accurate and complete, i.e. that bugs in the reference implementation don’t become de-facto spec.
If compression ratio is really important then I think you need a methodology for evaluating different alternatives over a good test corpus and actually measure the impact of different alternatives. We’ve all seen formats that tried to introduce DIY compression based on intuitions and made some really bad decisions (hello DWARF!). You’ll want to determine the maximum set of ASCII symbols you can use, and use them all. You probably should also figure out exactly what “human readability” means to you and push right up against that boundary.