I use something simpler in my config that runs on BufWritePost:

silent !svgo %

I wrote that right after reading this koan:

Master Wq and the Markdown acolyte

A Markdown acolyte came to Master Wq to demonstrate his Vim plugin.

“See, master,” he said, “I have nearly finished the Vim macros that translate Markdown into HTML. My functions interweave, my parser is a paragon of efficiency, and the results nearly flawless. I daresay I have mastered Vimscript, and my work will validate Vim as a modern editor for the enlightened developer! Have I done rightly?”

Master Wq read the acolyte’s code for several minutes without saying anything. Then he opened a Markdown document, and typed:

:%!markdown

HTML filled the buffer instantly. The acolyte began to cry.

https://blog.sanctum.geek.nz/vim-koans/