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They also have satori, which will run anywhere it seems.

That seems to generate svgs which as far as I know no site will show og embeds with svgs for some reason

I use svgs for my websites og images, haven't come across an issue with embeds on most platforms.

There are already a lot of choices to convert SVGs to PNGs, what Vercel uses is https://github.com/yisibl/resvg-js. And Sharp is another good option.