Something you maybe not know about #WebP is that it supports a #lossless mode as well, very similar to #PNG. To be clear, the lossless mode is not to set quality to 100%, but is AFAIK an unrelated image compression algorithm. Unfortunately, much of the software that offer to save images as WebP only offer the #lossy mode, but on the #commandline using WebP's own #cwebp, you can switch to lossless mode with the flag of the same name. WebP image viewers such as modern #webbrowsers have no problems with lossless WebP as the decoding is typically done by the Google-provided library that supports all features.
So, test to convert your PNG files to lossless WebP and check what your gains in compression ratio are!