WebP offers basically no advantage over good ol’ JPG in lossy compression of photos, which was the use case for this site.

I hear you on Core Web Vitals, but most sites will have a long list of more impactful changes to make before they could expect to see any measurable rank improvement by switching from JPG to WebP for photos.

That's shaving half a hair, isn't it?

Hence the initial “nitpicking” in my post

There are other ways of getting traffic besides Google. Sure, tweak your site to please their algorithm and achieve better SEO, but always be scouting for other traffic sources.