I really appreciated this article. It would be great to see a smaller svg graphics standard. I think SlimSVG is a bad name because it implies its still SVG. It should set its self apart. Just my 2 cents.
Concerning the simple use of standard shapes, you could restrict yourself to simple shapes within SVG : thus youd already have myriads of existing implementations and tooling, it would be mostly understandable by humans, short and writable by hand/simple scripts.
You could even call them mobile SVG Tiny and Basic : it exists since 2009. https://www.w3.org/TR/2003/REC-SVGMobile-20030114/ . That would be a good start.