btw, I am doing the exact same thing on bitcoininflationindex.com. I use netlify for my frontend and use their "On-demand builders" to generate images which are cached on the edge.

https://docs.netlify.com/configure-builds/on-demand-builders/

Works perfect for this use-case. You could spin up a netlify microservice specifically for this purpose and run it separate from the site.

Here is my code for reference: https://gist.github.com/theshanergy/42fd16010cc968b47f93df1a5413a6f1